Musnad Ahmad 1153:726

Abu Sa’id narrated to us, he said: ‘Abd al-Rahman ibn Abi al-Rijal narrated to us, he said: I heard my father narrating from ‘Amrah, from ‘Aishah, that the Prophet, peace and blessings of Allah be upon him, swore that he would not enter upon his wives for a month. When twenty-nine days of the month had passed, he came to enter. I said to him: "Did you not swear for a month?" He said: "The month is twenty-nine [days]."