Musnad Abu Ya'la al-Mawsili 115:363

Ubayd Allah narrated to us, Mu'adh narrated to us, from his father, from Qatadah, from Anas, that the Prophet (peace and blessings of Allah be upon him) used to rotate among his wives during the hour of the night and the day, and they were eleven. I said to Anas, "Was he capable of that?" He said, "We used to say that he was given the strength of thirty."