Abdullah bin Shaddad bin al-Had narrates from his father (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) came out to us carrying his grandson, Hasan or Husayn (may Allah be pleased with them both). He (peace and blessings be upon him) went forward to lead the prayer and set him down near his right foot. When the Messenger of Allah (peace and blessings be upon him) prostrated and made it very long, I raised my head and saw that the Messenger of Allah (peace and blessings be upon him) was in prostration and the child was riding on his (peace and blessings be upon him) back. I went back into prostration. When the Messenger of Allah (peace and blessings be upon him) finished the prayer with the salam, the people said: "O Messenger of Allah (peace and blessings be upon him)! You made a very long prostration in this prayer; you do not usually prostrate for that long. Were you given some command during the prayer, or was revelation coming to you?" He (peace and blessings be upon him) said: "None of that happened. Rather, my son was riding on me, and I did not like to hurry so that he could not fulfil his wish."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3423