Musnad Ahmad 31:603

Abu Nu'aym narrated to us, Sufyan narrated to us, from Yazid ibn Abi Ziyad, from Abd al-Rahman ibn Abi Nu'm, from Abu Sa'id, who said: The Messenger of Allah (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."