Hadith 2178
Sayyiduna Umar (may Allah be pleased with him) narrates that when the Messenger of Allah (peace and blessings be upon him) separated himself from his pure wives (for one month), I said, "O Messenger of Allah, you have stayed in the upper room for twenty-nine days (the month has not yet been completed)." So the Messenger of Allah (peace and blessings be upon him) said: "Indeed, a month can also be of twenty-nine days."