Musnad Ahmad 30:1357

Yunus narrated to us, Layth narrated to us, from Ibn ‘Ajlan, from Sumayy, the freed slave of Abu Bakr, from Abu Salih, from Abu Hurayrah, who said: "The companions of the Prophet, peace and blessings of Allah be upon him, complained to him about the difficulty of prostration upon them when they spread out [their arms]. He said: 'Seek help from the knees.'" Ibn ‘Ajlan said: "And that is for him to place his elbow upon his knee if the prostration is long and he becomes weary."