Ibn ‘Umar reported Allah's Messenger ﷺ as saying, “We are an unlettered people who can neither write nor count. The month is thus and thus and thus,” drawing in his thumb when he said it the third time.* Then he said, “The month is thus and thus and thus,” meaning a complete thirty days. He meant alternately twenty-nine and thirty. *Spreading out the ten fingers the first two times and nine the third time. (Bukhari and Muslim.)