Proclus, Chrestomathia 178–184
“Then Achilles killed Thersites because he was mocked by him when he reproached him, claiming he loved Penthesileia. A conflict arose among the Achaeans over the murder of Thersites. After that Achilles went sailing to Lesbos where, after he made a sacrifice to Apollo, Artemis and Leto, he was cleansed of the murder by Odysseus.”
καὶ Ἀχιλλεὺς Θερσίτην ἀναιρεῖ λοιδορηθεὶς πρὸς αὐτοῦ καὶ ὀνειδισθεὶς τὸν ἐπὶ τῆι Πενθεσιλείαι λεγόμενον ἔρωτα. καὶ ἐκ τούτου στάσις γίνεται τοῖς Ἀχαιοῖς περὶ τοῦ Θερσίτου φόνου. μετὰ δὲ ταῦτα Ἀχιλλεὺς εἰς Λέσβον πλεῖ, καὶ θύσας Ἀπόλλωνι καὶ Ἀρτέμιδι καὶ Λητοῖ καθαίρεται τοῦ φόνου ὑπ᾿ Ὀδυσσέως.
In some traditions, Penthesileia bore Achilles a child before she died.
Cf. Apollodorus, Epitome E 5
“…And later on, [Penthesileia] died at Achilles’ hands and he killed Thersites who was mocking him after her death because he had loved the Amazon.”
εἶθ᾿ ὕστερον θνήσκει ὑπὸ Ἀχιλλέως, ὅστις μετὰ θάνατον ἐρασθεὶς τῆς Ἀμαζόνος κτείνει Θερσίτην λοιδοροῦντα αὐτόν.
Quintus Smyrnaeus fleshes out some of the story in his Posthomerica: see the post on maicar.com.