Musnad Ahmad 830:85

'Affan narrated to us, Hammam narrated to us, Ziyad al-A'lam informed us, from al-Hasan, from Abu Bakrah, that he entered the mosque while the Prophet, peace and blessings of Allah be upon him, was bowing (in ruku'), so he bowed before he reached the row. The Prophet, peace and blessings of Allah be upon him, said to him: "May Allah increase you in eagerness, but do not do it again."