Musnad Abu Ya'la al-Mawsili 117:154

Muhammad ibn Abi Bakr al-Muqaddami narrated to us, Nuh ibn Qays narrated to us, Muhammad ibn Dhakwan narrated to us, from Thabit, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) would prostrate, and al-Hasan or al-Husayn would come and mount his back, so he would prolong the prostration. It was said, "O Prophet of Allah, you prolonged the prostration." He said, "My son mounted me, and I disliked to hasten him."