Musnad Ahmad 30:2507

Yahya narrated to us, from Ubaydullah, who said: Sa'id ibn Abi Sa'id narrated to me, from his father, from Abu Hurayrah, who said: A man entered the mosque and prayed, while the Prophet, peace and blessings of Allah be upon him, was in the mosque. Then he came to the Prophet, peace and blessings of Allah be upon him, and greeted him. He returned the greeting and said: "Go back and pray, for you have not prayed." He went back and did that three times. He said: "By the One who sent you with the truth, I do not know any better than this, so teach me." He said: "When you stand for prayer, say the takbir (Allahu Akbar), then recite what is easy for you of the Quran. Then bow until you are at ease while bowing, then rise until you are standing upright, then prostrate until you are at ease while prostrating, then rise until you are at ease while sitting. Then do that in all of your prayer."