Musnad Ahmad 2589

It was narrated that Salih, the freed slave of at-Taw'amah said: I heard Ibn 'Abbas (رضي الله عنه) say: A man asked the Prophet (ﷺ) about something to do with prayer, and the Messenger of Allah (ﷺ) said to him: "Let the water go between your fingers and toes - i.e., do wudoo’ properly.” And among the things he said was: "When you bow, put your hands on your knees until you pause and settle in that position. When you prostrate, press your forehead into the ground until it is firmly placed on the ground.”