Musannaf Ibn Abi Shayba 27:536

Abu Bakr ibn Ayyash narrated to us, from Asim, from Zirr, who said: Al-Hasan and Al-Husayn used to jump upon the back of the Messenger of Allah, may Allah bless him and grant him peace, while he was praying. The people began to move them away, so the Prophet, may Allah bless him and grant him peace, said: "Leave them both. May my father and mother be sacrificed for them. Whoever loves me, let him love these two."