Sahih Muslim | Book 6 | Hadith Number 2374

Narrated by Ibn Umar (Allah be pleased with both of them)
Ibn Umar (Allah be pleased with both of them) reported Allah's Messenger (may peace be upon him) as saying: The month is thus, and thus, and thus, and he flapped his hands with all their fingers twice, but at the third turn, folded his right thumb or left thumb (in order to give an idea of twenty-nine).