Hadith 1921
Sayyiduna Umar bin Khattab (may Allah be pleased with him) narrates that when the Messenger of Allah (peace and blessings of Allah be upon him) separated himself from his wives, I said, "O Messenger of Allah, you have stayed in the upper room for twenty-nine days (while you had chosen separation for a month)." So the Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, a month can also be of twenty-nine days."