Sahih Bukhari Volume 8, Book 78, Hadith Number 675.

Narated By Anas : Allah’s Apostle took an oath for abstention from h is wives (for one month), and during those days he had a sprain in his foot. He stayed in a Mashrubah (an upper room) for twenty-nine nights and then came down. Then the people said, “O Allah’s Apostle! You took an oath for abstention (from your wives) for one month.” On that he said, A month can be of twenty-nine days.”