Musnad Ahmad 30:1945

Aswad ibn Amir narrated to us, Dhawwad Abu al-Mundhir narrated to us, from Layth, from Mujahid, from Abu Hurairah, who said: I never woke up in the night except that I found the Prophet, peace and blessings of Allah be upon him, praying. He said: So he prayed, then he said: "Do you have pain?" I said: No. He said: "Stand and pray, for indeed in prayer there is a cure."