Chapter 30: Fasting
Narrated Anas: Allah' s Messenger (ﷺ) vowed to keep aloof from his wives for one month, and he had dislocation of his leg.
So, he stayed in a Mashruba for 29 nights and then came down. Some people said, "O Allah' s Messenger (ﷺ)!
You vowed to stay aloof for one month," He replied, "The month is of 29 days."
Sahih al-Bukhari 1911