Musnad Ahmad 30:2280

Qutaybah narrated to us, he said: Ya'qub narrated to us, from Ibn 'Ajlan, from Sumayy, from Abu Salih, from Abu Hurayrah, that he said: "The people complained to the Messenger of Allah, may Allah bless him and grant him peace, about the opening between the elbows [during prostration], so the Messenger of Allah, may Allah bless him and grant him peace, commanded them to seek assistance with the knees."