Musnad Ahmad 937:20

Yahya ibn Adam narrated to us, Zuhayr narrated to us, from Abu Ishaq, from al-Mukhariq, who said: We went out as pilgrims, and when we reached al-Rabadhah, I said to my companions: "Go ahead," and I stayed behind. I came to Abu Dharr while he was praying, and I saw him prolonging the standing, and increasing the bowing and prostrating. I mentioned that to him, and he said: "I have not failed to do my best. Indeed, I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Whoever bows a bowing or prostrates a prostration, Allah raises him a degree by it, and removes a sin from him by it.'"