Musnad Ahmad 30:2117

Musa ibn Dawud narrated to us, he said: Dhawwad ibn 'Ulbah narrated to us, from Layth, from Mujahid, from Abu Hurayrah, who said: The Prophet, peace and blessings of Allah be upon him, was suffering from pain. He said: So I prayed, then I came and sat with him. He said: "O Abu Hurayrah, is your stomach aching?" I said: "No, O Messenger of Allah." He said: "Pray, for indeed in prayer there is a cure."