Musannaf Ibn Abi Shayba 16:787

Abu Bakr narrated to us, he said: Abu Bakr ibn Ayyash narrated to us, from Abd al-Aziz ibn Rufay’, from Qays, the freed slave of Khabbab, who said: I entered upon al-Hasan and al-Husayn, and they were both dyeing [their hair/beards] with black.