Musannaf Ibn Abi Shaybah 9863 — 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, raising his hand, he held back one finger, that is, he counted to twenty-nine.