Musnad Ahmad 30:1803

Qutaybah ibn Sa'id narrated to us, he said: Bakr ibn Mudar narrated to us, from Ibn al-Had, from Muhammad ibn Ibrahim, from Abu Salamah, from Abu Hurairah, that he heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Do you see, if there were a river at the door of one of you, and he bathed in it five times every day, what do you say? Would any of his filth remain?" They said: "Nothing of his filth would remain." He said: "That is the likeness of the five prayers; Allah erases the sins by them."