Musnad Abu Ya'la al-Mawsili 151:51

Abu Bakr narrated to us, Ubayd Allah ibn Musa narrated to us, from Ali ibn Salih, from Asim, from Zirr, from Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to pray. When he would prostrate, al-Hasan and al-Husayn would jump onto his back. When they (the people) wanted to stop them, he signaled to them to leave them alone. When he finished the prayer, he placed them in his lap and said: "Whoever loves me, let him love these two."