It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "We are an unlettered nation; we do not know calculation. Sometimes the month is like this, like this, and like this," and the third time the Prophet (peace and blessings be upon him) folded his thumb, "and sometimes it is like this, like this, and like this," meaning a complete thirty days.