From Sahih Bukhari (Hadith 5289):
Narrated Anas bin Malik:Allah's Messenger (ﷺ) took an oath that he would abstain from his wives, and at that time his leg had been sprained (dislocated). So he stayed in the Mashruba (an attic room) of his for 29 days. Then he came down, and they (the people) said, "O Allah's Messenger (ﷺ)! You took an oath to abstain from your wives for one month." He said, "The month is of twenty nine days
Reference: Bukhari 5289