Musannaf Ibn Abi Shayba 27:538

Waki' narrated to us, from Sufyan, from Yazid ibn Abi Ziyad, from Ibn Abi Nu'm, from Abu Sa'id, who said: He (meaning the Prophet, peace and blessings of Allah be upon him) said: "Al-Hasan and Al-Husayn are the masters of the youth of the people of Paradise."