Musnad Ahmad 32:737

'Abd al-Razzaq narrated to us, Malik narrated to us, Ishaq ibn 'Abd Allah ibn Abi Talha narrated to us, from Anas ibn Malik, that his grandmother, Mulaykah, invited the Prophet, peace and blessings of Allah be upon him, for food she had prepared for him. He said: So he ate, then he said: "Stand up, so that I may pray for you." He said: So I stood up to a mat of ours that had turned black from the length of time it had been used, and I sprinkled it with water. Then the Messenger of Allah, peace and blessings of Allah be upon him, stood, and I and the orphan stood in a row behind him, and the old woman stood behind us. He prayed two rak'ahs for us, then he departed.