Musannaf Ibn Abi Shayba 4:735

Ibn Numayr narrated to us, from Hajjaj, from Salamah ibn Kuhayl, from a man from Banu Sulaym, from Ibn Abbas, from Umar, who said: The Prophet (peace and blessings of Allah be upon him) secluded himself from his wives for a month. When twenty-nine days had passed, Gabriel came to him and said: "The month has been completed, and you have fulfilled your oath."