Musannaf Ibn Abi Shayba 27:553

Yazid ibn Harun narrated to us, he said: Jarir ibn Hazim informed me, from Muhammad ibn Abdullah ibn Abi Ya'qub, from Abdullah ibn Shaddad, from his father, who said: The Messenger of Allah (may Allah bless him and grant him peace) was called for prayer, so he went out while carrying Hasan or Husayn. He placed him beside him, then he prostrated during his prayer, a prostration in which he prolonged. My father said: I raised my head from among the people, and behold, the boy was on the back of the Messenger of Allah (may Allah bless him and grant him peace). So I returned my head and prostrated. When the Messenger of Allah (may Allah bless him and grant him peace) performed the taslim, the people said to him: "O Messenger of Allah, you performed a prostration in this prayer of yours that you were not accustomed to performing. Was revelation coming to you?" He said: "No, but my son rode me, and I disliked to hasten him until he had fulfilled his need."