Mishkat al-Masabih #1880

Ibn ‘Umar reported God’s messenger 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.)