Sahih Bukhari Volume 7, Book 62, Hadith Number 130.

Narrated By Um Salama : The Prophet took an oath that he would not enter upon some of his wives for one month. But when twenty nine days had elapsed, he went to them in the morning or evening. It was said to him, “O Allah’s Prophet! You had taken an oath that you would not enter upon them for one month.” He replied, “The month can be of twenty nine days.”