Sahih Bukhari Volume 003, Book 031, Hadith Number 134.

Narated By Um Salama : The Prophet vowed to keep aloof from his wives for a period of one month, and after the completion of 29 days he went either in the morning or in the afternoon to his wives. Someone said to him "You vowed that you would not go to your wives for one month." He replied, "The month is of 29 days."