Abū ʿUthmān ʿAmr ibn Baḥr al-Jāḥiẓ (died 868/869 CE at Basra),"Exploits of the Turks" p. 646 The Case for Khorasān:
Translation from C. T. Harley Walker, "Jahiz of Basra to Al-Fath Ibn Khaqan on the 'Exploits of the Turks and the Army of the Khalifate in General,'" The Journal of the Royal Asiatic Society of Great Britain and Ireland (October 1915), pp. 631-697 https://www.jstor.org/stable/25189369 or https://archive.org/details/in.ernet.dl ... 5/mode/2upAnd we have drums that strike terror into the foe and large banners; and we possess coats of mail and bells and epaulettes and long hair and twisted sheaths and curled moustaches and muslin caps and Shihry steeds. And the axe and the battle-axe is on our pack saddles, and the daggers are at our waists. And we know how to hang up our swords and to sit elegantly on our horses' backs. And we have shouts that make pregnant women deliver prematurely. And there is not  in the world any wonderful craft of culture and wisdom and science and engineering and music and workmanship and law and tradition, in which Khorasān has been concerned, but she has beaten the experts and surpassed the savants. And we make armour of felt (libd), and have stirrups and breastplates. And we possess among our institutions for training and practice and preparation for war and training and practice in driving back the foe and attacking him with the spear, and in turning back our horses after flight, such games as 'Dābbūq' and leaping on our steeds, when young; and polo, when grown up. Then we practise throwing at the bird at rest and at targets and at the bird of prey on the wing. So we deserve better to be preferred and have the better right to the first place."
- Thucydides 4.34.3 τό τε ἔργον ἐνταῦθα χαλεπὸν τοῖς Λακεδαιμονίοις καθίστατο: οὔτε γὰρ οἱ πῖλοι ἔστεγον τὰ τοξεύματα, δοράτιά τε ἐναπεκέκλαστο βαλλομένων "And now the Lacedaemonians began to be sorely distressed, for their felts (piloi) did not protect them against the arrows, and the points of the javelins broke off where they struck them."
- Agatharchides of Knidos (Nubia, 3rd century BCE) "For the war against the Aithiopians Ptolemy (II of Egypt) recruited 500 cavalrymen from Greece. To those who were to fight in the front ranks and to be the vanguard - they were a hundred in number - he assigned the following form of equipment. For he distributed to them and their horses garments of felt (stolas piletas), which those of that country (hoi kata ten choran; "the natives of the country" in Burstein) call kasas, that conceal the whole body except for the eyes."
- Caesar, Civil Wars 3.44.6 (1st century BCE): from Armour in Texts
Pompey sent archers and slingers, of which he had a great number, into his own positions, and they wounded many of our men, and a great fear of arrows came upon them, until almost all of the soldiers had made tunics or coverings out of felts (ex coactis) or patchworks or leathers, with which they were protected from missiles.
- Pliny, Historia Naturalis 8.73 / 8.192 (1st century CE summarizing everything a Roman senator with no life could read in Greek or Latin) lanae et per se coactae vestem faciunt et, si addatur acetum, etiam ferro resistunt "And from wool rubbed against itself they make a garment and, if vinegar is added, it can even resist iron" Lacus Curtius
- De Rebus Bellicis (Western Roman Empire, 4th or 5th century CE): This list of clever military devices describes a felt garment which is either worn under iron armour (section 1-3) or on its own (section 4 and the Carolingian illustration).
You can find the Latin text in E. A. Thompson, A Roman reformer and inventor, being a new text of the treatise De rebus bellicis (Oxford: Clarendon Press, 1952)XV. Exposition of the Thoracomachus
 Among all the things which antiquity saw fit to provide to posterity for the use of war, the thoracomachus is marvelously useful for lightening the burden of arms on the body and placing under rough things.  For this is a kind of garment, which is made from felt (coactilis) to the measure and for the protection of the human breast, from soft wools. Worry of fear, that most clever mistress, made it, so that having first put it on the coat of mail (lorica) or lamellar armour (clivanus) or similar things to these would not pain the vulnerable body with the roughness of weight.  To be sure, lest the same thoracomachus, struck by rain, should begin to weigh burdensomely, it is wise to put on top a garment, preferably made of Libycian skins, in the same shape as that particular thoracomachus.  And so, as we say, this thoracomachus having been put on (which takes for a name this Greek term for the protection of the body), socci as well, that is shoes / hose, and iron greaves having been put on, a helmet (galea) having been put on top and a shield or sword fitted to the side, having grasped lances, the soldier shall be armed in full to undergo a fight on foot.
- Chapter 16.9 of the anonymous on generalship (East Roman, 9th century?) in Hermann Köchly and Wilhelm Rüstow, ed. and tr., Griechische Kriegsschriftsteller. Vol. 2, Pt. 5: Des Bzyantiner Anonymus Kriegswissenschaft (Leipzig: Wilhelm Engelmann Verlag, 1855) pp. 104, 105 https://archive.org/details/griechischekrieg01kc/ There is supposed to be a complete English translation in George T. Dennis, Three Byzantine Military Treatises (Washington, DC: Dumbarton Oaks Research Library and Collection, 2009), Timothy Dawson has his own take on these passages.
In addition to offensive weapons and shields, the heavy infantry should have helmets (perikephaleiai) and body armour (thorakes) and greaves (periknemides). If, however, sufficient equipment is not available, then at least the first two ranks should have these, and the rest should have kaftans (zabai) and body armour and helmets of felt (pilos) and hide (βυρσίς). By the 10th century, East Roman or Byzantine texts say "cotton or rough silk" instead of felt.
- Ekkehard IV, Casus Sancti Galli ("Events at St. Gall": the author died in the middle of the 11th century) MGH SS 2 p. 104 from our MyArmoury chat last year
Nam Ungri, auditis tempestatibus regni, Noricos rabidi invadunt et vastant, Augustaque diu obsessa, precibus Uodalirici episcopi, sanctissimi quidem inter omnes tunc temporis viri, repulsi, Alemanniam nemine vetante turmatim pervadunt. At Engilbertus, quam idoneus ad mala toleranda quidem fuerit, impiger ostendit. Nam malis his immenentibus militum suorum unoquoque pro semetipso sillicito, validores fratrum arma sumere iubet, familiam roborat, ipse velut Domini gigans lorica indutus, cucullam superinduens et stolam, ipsos eadem facere iubet: 'Contra diabolum,' ait, 'fratres mei, quam hactenus animis in Deo confisi pugnaverimus, ut nunc manibus ostendere valeamus, ab ipso petamus.' Fabricantur spicula, piltris loricae fiunt, fundibula plectuntur, tabulis compactis et wannis scuta simulantur, sparrones (= modern German Sperr) et fustes acute focis praedurantur.
Sed primo fratrum quidam et familiae, famae increduli, fugere nolunt. Eligitur tamen locus velud a Deo in promptu oblatus, ad arcem parandam circa fluvium Sint-tria-unum; quem sanctus Gallus quondam sanctae Trinitatis amore de tribus fluviis in unum confluentibus sic equivocasse fertur. Praemunitur in artissimo collo vallo, et silva excisis locus, fitque castellum, ut sanctae Trinitati decuit, fortissimum. Convehuntur raptim, quaeque essent necessaria. Haec in vita Wiboradae per scriptorem eius minus dicta, a fratribus qui haec noverant docti perstrinximus. ...
"For the Huns (ie. Magyars), having heard the disorders of the realm, savagely invaded and ravaged Noricum (in 926), and besieged Augusta (ie. Augsburg) for a long time; driven off by the prayers of bishop Woldaliric, certainly the most holy man of that time, they penetrated the forbidden Alemannian forest in squadrons." Abbot Engilbert of St. Gall prepared the "stronger of the brothers" and the hangers-on to defend themselves, so "darts are made, body armour created from felts (piltris loricae fiunt), slings woven, with joined tablets and twigs shields are imitated, bolts and cudgels are hardened to a sharp point in the hearth" but a few lines later they are retreating to a high place called Siteruna (Sint-tria-unum "Three shall be one" explains the chronicler, "that is folk etymology for an honest German word!" says the editor) with all the most necessary things before the scouts of the Huns arrive.
- Anonymus de Gestis Herwardi Saxonis (written in England c. 1107-1131): Some footsoldiers from Scaldemariland are armed "with felt togas soaked with pitch and resin and thus"(cum feltreis togis pice et resina atque in thure intinctis)
- Bahāʾ al-Dīn, Life of Saladin (describes events in 1191, written slightly later): in one manuscript, frankish infantry are said to wear a libd "felt garment" and a long coat of mail, in another manuscript they wear an "iron cuirass" and a long coat of mail
Edit 2023-08-13: Added the anonymous on generalship. The Old French verb affeutrer "to felt; to prepare for combat" could derive from a felt underarmour garment but the only evidence is the etymology (David Nicolle, Crusader Warfare, 2 vols. (London: Hambledon Cntinuum, 2007) vol. 1 p. 98