Musannaf Ibn Abi Shaybah 9860 — How many days there are in a month

It is narrated from Sayyiduna 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 this much, this much and this much." The third time, he folded his thumb (that is, he counted to twenty-nine). Then he said: "The month is this much, this much and this much." This time he counted the full thirty.