Musannaf Ibn Abi Shaybah

It is narrated from Hazrat Ibn Umar (may Allah be pleased with them both) that the Noble Prophet (peace and blessings be upon him) said that a month consists of twenty-nine days. Then he fully opened both his hands twice, and the third time kept his thumb closed. When Hazrat Aisha (may Allah be pleased with her) heard the narration of Hazrat Ibn Umar (may Allah be pleased with them both), she said, "May Allah have mercy on Abu Abdur Rahman. In fact, the Messenger of Allah (peace and blessings be upon him) had left his wives for a month. He returned after twenty-nine days, so the people said, 'O Messenger of Allah! You had taken an oath of separation for a month.' The Prophet (peace and blessings be upon him) said, 'A month is sometimes twenty-nine days.'"