Musnad Abu Ya'la al-Mawsili 161:16

Muhammad ibn al-Faraj narrated to us, Muhammad ibn al-Zibriqan narrated to us, Muhammad ibn ‘Ajlan narrated to us, from Sumayy, from Abu Salih, from Abu Hurayrah, that some of the companions of the Messenger of Allah (may Allah bless him and grant him peace) said: "O Messenger of Allah, spreading the hands [in prostration] is difficult for us in prayer." So he commanded them to seek assistance with the knees.