Musnad Ahmad 5:800

Zakariya ibn 'Adi narrated to us, 'Ubayd Allah ibn 'Amr informed us, from 'Abd Allah ibn Muhammad ibn 'Aqil, from Muhammad ibn 'Ali, from 'Ali, may Allah be pleased with him, who said: When al-Hasan was born, I named him Hamzah. Then, when al-Husayn was born, I named him after my uncle, Ja'far. He said: So the Messenger of Allah, peace and blessings of Allah be upon him, called me and said: "I have been commanded to change the names of these two." I said: Allah and His Messenger know best. So he named them al-Hasan and al-Husayn.