Musnad Ahmad 31:397

Husayn ibn Muhammad narrated to us, Ayyub ibn Jabir narrated to us, from Abdullah ibn Ismah al-Hanafi, from Abu Sa’id al-Khudri, who said: A man prayed behind the Prophet, peace and blessings of Allah be upon him, and he began to bow before he [the Prophet] bowed, and he would rise before he [the Prophet] rose. When the Prophet, peace and blessings of Allah be upon him, finished the prayer, he said: "Who did this?" He said: "I did, O Messenger of Allah. I wanted to know whether you would know that or not." He said: "Beware of the deficiency of the prayer. When the Imam bows, then bow, and when he rises, then rise."