Musnad Ahmad 31:15

Muhammad ibn Abdullah al-Zubayri narrated to us, Yazid ibn Mardanbah narrated to us, he said: Ibn Abi Nu'm narrated to us, from Abu Sa'id al-Khudri, he 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."