Musnad Abu Ya'la al-Mawsili 129:108

Ibrahim ibn al-Hajjaj narrated to us, Hammad narrated to us, from Humayd, from Anas, that the Messenger of Allah (may Allah bless him and grant him peace) swore an oath to abstain from his wives for a month. He was in a loft of his. His foot became dislocated, so his companions came to visit him. He prayed with them while sitting, while they were standing. Then they came for another prayer, and he sat and they stood, so he gestured to them to sit, and they prayed behind him while they were sitting. When twenty-nine nights had passed, he came down to them. It was said, "O Messenger of Allah, only twenty-nine nights have passed." He said, "Indeed, the month is twenty-nine."