Musnad Ahmad 1316:1

Yazid ibn Harun narrated to us, he said: Jarir ibn Hazim informed us, he said: Muhammad ibn Abi Ya'qub narrated to us, from Abdullah ibn Shaddad, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) came out to us during one of the two evening prayers—either Dhuhr or Asr—while he was carrying al-Hasan or al-Husayn. The Prophet (peace and blessings of Allah be upon him) stepped forward and placed him down, then he said the takbir for the prayer and prayed. He performed a prostration during his prayer and prolonged it. My father said: I raised my head, and behold, the child was on the back of the Messenger of Allah (peace and blessings of Allah be upon him) while he was prostrating, so I returned to my prostration. When the Messenger of Allah (peace and blessings of Allah be upon him) finished the prayer, the people said: "O Messenger of Allah, you performed a prostration during the prayer and prolonged it, until we thought that something had happened, or that revelation was being sent down to you?" He said: "None of that happened, but my son had mounted me, and I disliked to rush him until he had fulfilled his need."