It is narrated from Sayyiduna Ali (may Allah be pleased with him) that he had named his eldest son Hamzah, and had named Husayn Ja'far after his (Husayn's) paternal uncle; then the Messenger of Allah (peace and blessings be upon him) named the two of them Hasan and Husayn.
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2767