Mukhtasar Muslim

Narrated from Umm al-Mu’minin Umm Salamah (may Allah be pleased with her) that the Noble Prophet (peace and blessings of Allah be upon him) took an oath that he would not go to his wives for a month. (But) when twenty-nine (29) days had passed, the Prophet (peace and blessings of Allah be upon him) went to his wives in the morning or evening. He was told, “O Messenger of Allah (peace and blessings of Allah be upon him), you had taken an oath that you would not come to us for a month.” The Prophet (peace and blessings of Allah be upon him) replied, “A month can also be of twenty-nine days.”

Sayyiduna Ibn Umar (may Allah be pleased with them both) narrates from the Noble Prophet (peace and blessings be upon him) that he said: We are an unlettered nation; we neither write nor calculate. The month is like this, like this, and like this. And the third time, he folded his thumb. And the month is like this, like this, and like this. That is, it completes thirty days.