Mukhtasar Muslim

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.