Musnad Abu Ya'la al-Mawsili 152:300

Abu Khaythama narrated to us, Ubayd Allah ibn Musa narrated to us, Ali ibn Salih narrated to us, from Asim, from Zirr, from Abdullah ibn Mas’ud, who said: The Messenger of Allah (may Allah bless him and grant him peace) was praying. When he prostrated, 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."