Musnad Ahmad 31:777

Abu Nu'aym narrated to us, Sufyan narrated to us, from Yazid ibn Abi Ziyad, from Ibn Abi Nu'm, from Abu Sa'id al-Khudri, 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."