The Prophet (peace and blessings be upon him) said: "We are an unlettered people; we neither write nor do we know calculation. The month is like this, like this and like this," and (the third time) he (peace and blessings be upon him) folded one finger (meaning twenty-nine days), "and it is like this, like this and like this," meaning thirty days.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13287