Musnad Ahmad 25:745

Sulayman ibn Dawud al-Hashimi narrated to us, Abd al-Rahman ibn Abi al-Zinad narrated to us, from Musa ibn Uqbah, from Salih, the freed slave of al-Taw'amah, who said: I heard Ibn Abbas saying: A man asked the Prophet, peace and blessings of Allah be upon him, about something regarding the matter of prayer. The Messenger of Allah, peace and blessings of Allah be upon him, said to him: "Interlace the fingers of your hands and your feet"—meaning performing wudu thoroughly. And among what he said to him was: "When you bow, place your palms on your knees until you are at ease"—and al-Hashimi said once: "until you both are at ease"—"and when you prostrate, settle your forehead upon the ground until you feel the firmness of the ground."