Musnad Abu Ya'la al-Mawsili 209:109

Abu Khaythama narrated to us, Abu ‘Asim al-Dahhak ibn Makhlad narrated to us, from Ibn Jurayj, from Yahya ibn ‘Abd Allah ibn Sayfi, who said: ‘Ikrimah ibn ‘Abd al-Rahman ibn al-Harith ibn Hisham informed me that Umm Salamah informed him: That the Prophet (peace and blessings of Allah be upon him) swore that he would not enter upon some of his wives for a month. When twenty-nine days had passed, he went out in the morning or the evening. She said: I said, "O Messenger of Allah, you swore that you would not enter [for a month]." He said, "The month can be twenty-nine days."