Musnad Ahmad 8:96

Harun ibn Ma'ruf narrated to us, and Abdullah said: I heard it myself from Harun, Abdullah ibn Wahb narrated to us, Makhramah narrated to me, from his father, from Amir ibn Sa'd ibn Abi Waqqas, who said: I heard Sa'd and a group of the companions of the Messenger of Allah, peace and blessings of Allah be upon him, saying: There were two brothers during the time of the Messenger of Allah, peace and blessings of Allah be upon him, and one of them was better than the other. The one who was better of the two passed away, then the other lived for forty nights after him, then he passed away. The superiority of the first over the second was mentioned to the Messenger of Allah, peace and blessings of Allah be upon him, so he said: "Was he not praying?" They said: "Yes, O Messenger of Allah, and he was not bad." He said: "What makes you know what his prayer attained for him?" Then he said at that: "The likeness of the prayer is only like a flowing river at a man's door, deep and fresh, in which he plunges five times every day. What do you think that leaves of his filth?"