Musannaf Ibn Abi Shayba 9:1017

Affan narrated to us, from Hammad ibn Salamah, from Ali ibn Zayd, from al-Hasan regarding [the verse]: "Nor to exchange them for other wives," he said: Allah restricted him to his nine wives whom he had when he died. He said: Ali said, "I informed Ali ibn al-Husayn of that, and he said: 'He was permitted to marry.'"