Laudator Temporis Acti

Cicero, On Old Age 3.7 (tr. Cyrus R. Edmonds):

Equals with equals, according to the old proverb, most easily flock together.

pares autem vetere proverbio cum paribus facillime congregantur.

A. Otto, Die Sprichwörter und sprichwörtlichen Redensarten der Römer (Leipzig: B.G. Teubner, 1890), p. 264:

Cic. de sen. 3, 7 pares autem vetere proverbio cum paribus facillime congregantur (citiert von Quintil. 5, 11, 41). Amm. Marcell. 28,1, 53 ut solent pares facile congregari cum paribus. Macrob. 7, 7, 12 Similibus enim similia gaudent. Augustin de spir. et an. 14. Porphyr. zu Hor. ep. 1, 18, 89 ostendit similem similibus delectari. Cassiod. Var. 1, 4 ut se pares animi solent semper eligere. Isidor synon. 2, 44 similes enim similibus coniungi solent.*) Vgl. Liv. 1, 46, 7 contraxit celeriter similitudo eos, ut fere fit malum malo aptissimum. Hom. Od. 17, 218 αἰεὶ τὸν ὁμοῖον ἄγει θεὸς ὡς τὸν ὁμοῖον. Plato sympos. 18, 3 p. 195 B. ὁ παλαιὸς λόγος εὖ ἔχει, ὡς ὅμοιον ὁμοίῳ ἀεὶ πελάζει. Gorg. p. 510 B. Diogen. 5, 16. Apost. 12, 68. — Ter. Heaut. 419 Nos quoque senes est aequom senibus obsequi erinnert an die griechische vollständigere Form ἥλιξ ἥλικα τέρπει, γέροντα δὲ γέρων (Diogen. 5, 16). 'Gleich und gleich gesellt sich gern' (Düringsf. I n. 601). Vgl. Cic. de amic. 27, 101.

See also Renzo Tosi, Dictionnaire des sentences latines et grecques, tr. Rebecca Lenoir (Grenoble: Jérôme Millon, 2010), pp. 380-382 (#469: Ὡς αἰεὶ τὸν ὁμοῖον ἄγει θεὸς ὡς τὸν ὁμοῖον), and Erasmus, Adagia I ii 20 (Aequalis aequalem delectat). In English, "Birds of a feather flock together" — Bartlett Jere Whiting, Early American Proverbs and Proverbial Phrases (Cambridge: Belknap Press, 1977), pp. 31-32 (B177), and also his Proverbs, Sentences, and Proverbial Phrases from English Writings Mainly Before 1500 (Cambridge: Belknap Press, 1968), pp. 29 (B129: Every Beast loves its like), 164 (E171: Everything seeks its semblable), 210 (F574: Fowls come to their like), and 339-340 (L272: Like to like).