Mustadrak al-Hakim — Hadith 4831

Sayyiduna Abdullah bin Shaddad bin al-Had narrates from his father that the Messenger of Allah (peace and blessings be upon him) came out to us carrying one of his two sons, Hasan or Husayn. The Messenger of Allah (peace and blessings be upon him) stepped forward and placed him by his right foot, then the Messenger of Allah (peace and blessings be upon him) made a prostration (in the prayer) which he prolonged greatly. My father said: I raised my head from among the people and saw the Messenger of Allah (peace and blessings be upon him) in prostration, with the boy riding on his 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 you made a prostration such as you do not (usually) make. Were you commanded with something new, or was revelation coming down to you? He (peace and blessings be upon him) said: "None of that was the case; rather, my son was riding on me, and I disliked hurrying him down before he had fulfilled his need (for play)."
This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it.