Sayyiduna Shaddad bin al-Had (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) came out to us for one of the two daytime prayers, Zuhr or Asr, while carrying Sayyiduna al-Hasan or Sayyiduna al-Husayn (may Allah be pleased with them both). He (peace and blessings be upon him) went forward and placed him beside his right foot. Then the Messenger of Allah (peace and blessings be upon him) performed a prostration and prolonged it. I raised my head from among the people 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, so I went back into prostration. When the Messenger of Allah (peace and blessings be upon him) finished the prayer, the people said: "O Messenger of Allah (peace and blessings be upon him), in this prayer of yours you made a prostration that you never used to make before. Were you commanded to do so? Or was revelation coming down to you?" He (peace and blessings be upon him) said: "None of that happened; rather, my son (grandson) was riding on me, and I disliked hurrying him (off) until he had satisfied his wish."
Hadith Referenceالمعجم الكبير للطبراني / باب الشين / 7076