Musnad Ahmad 830:136

Abu Bakrah said: I came while the Prophet of Allah, peace and blessings of Allah be upon him, was bowing, and I was out of breath, so I bowed before reaching the row. When the Messenger of Allah, peace and blessings of Allah be upon him, finished the prayer, he said: "Which of you bowed before reaching the row?" I said: "I did." He said: "May Allah increase your eagerness, but do not do it again."