Musannaf Ibn Abi Shaybah

It is narrated from Hazrat Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: We are an unlettered nation; we neither write nor calculate. The month is like this, like this, and like this. On the third time, he folded his thumb (i.e., counted up to twenty-nine). Then he said: The month is like this, like this, and like this. This time, he counted up to thirty.