Musnad Ahmad 1153:41

'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from 'Urwah ibn al-Zubayr, from 'Aishah, that she said: The Messenger of Allah (peace and blessings of Allah be upon him) swore an oath that he would not enter upon his wives for a month. She said: So he remained for twenty-nine [days]. She said: I was the first whom he began with, so I said to the Prophet (peace and blessings of Allah be upon him): "Did you not swear for a month?" And I counted the days as twenty-nine. The Prophet (peace and blessings of Allah be upon him) said: "The month is twenty-nine."