Musnad Abu Ya'la al-Mawsili 3:238

'Isa ibn Salim narrated to us, 'Ubayd Allah ibn 'Amr narrated to us, from Ibn 'Aqil, from Muhammad ibn 'Ali, from 'Ali ibn Abi Talib: That he named his eldest son Hamzah, and he named al-Husayn after his uncle Ja'far. He said: So the Messenger of Allah (may Allah bless him and grant him peace) summoned 'Ali. When he came, he said: "You have changed the names of these two sons of mine." I said: "Allah and His Messenger know best." So he named them al-Hasan and al-Husayn.