Musnad Ahmad 30:2564

Muhammad ibn Ubayd narrated to us, he said: Al-A’mash narrated to us, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The likeness of these five prayers is only like a flowing river at the door of one of you, in which he bathes five times every day. So what does that leave of his filth?"