Musnad Abu Ya'la al-Mawsili 129:11

Zuhayr narrated to us, Yahya narrated to us, from Humayd, from Anas: That the Prophet (peace and blessings of Allah be upon him) swore an oath to abstain from his wives for a month, so he sat in a loft of his. His foot had become dislocated. He said: So they entered upon him, and the prayer time arrived, so he prayed with them while sitting, and they prayed while standing. Then he came down after twenty-nine [days]. They said: "You swore an oath for a month." He said: "The month is twenty-nine."