The Messenger of Allah ﷺ came out to us for one of the two afternoon prayers, either Zuhr or Asr, carrying Hasan or Husayn. The Prophet ﷺ went forward, set the child down, and then said the takbir to begin the prayer. During the prayer he made one prostration that he prolonged greatly.The narrator said, "I raised my head, and there was the boy on the back of the Messenger of Allah ﷺ while he was prostrating. So I returned to my prostration."When the Messenger of Allah ﷺ finished the prayer, the people said, "Messenger of Allah, you made a prostration during your prayer that you prolonged so much that we thought something had happened, or that revelation was being sent down to you."He replied, "None of that was the case. Rather, my son was riding on my back, and I disliked to hurry him until he had finished what he wanted."
قال حدثنا يزيد قال أخبرنا جرير بن حازم عن محمد بن أبي يعقوب
خرج علينا رسول الله ﷺ في إحدى صلاتي العشي الظهر أو العصر وهو حامل الحسن أو الحسين فتقدم النبي ﷺ فوضعه ثم كبر للصلاة فصلى فسجد بين ظهراني صلاته سجدة أطالها فقال إني رفعت رأسي فإذا الصبي على ظهر رسول الله ﷺ وهو ساجد فرجعت في سجودي فلما قضى رسول الله ﷺ الصلاة قال الناس يا رسول الله إنك سجدت بين ظهراني صلاتك هذه سجدة قد أطلتها فظننا أنه قد حدث أمر أو أنه قد يوحى إليك قال فكل ذلك لم يكن ولكن ابني ارتحلني فكرهت أن أعجله حتى يقضي حاجته
Its isnad is sahih. Muhammad bin Yaqub is named after his grandfather, and he is Muhammad bin Abdullah bin Abu Yaqub Taymi. He is considered trustworthy by everyone, and his hadith is accepted by all the major hadith compilers.Nasai related the hadith in 2/229, number 1141, in the chapter on prostration, on whether it is permissible for one sajdah to be longer than another.