Sahih Muslim | Book 6 | Hadith Number 2371

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 (i.e. pointing with his fingers thrice), and he held back his thumb at the third time (in order to show that it can also consist of twenty-nine days).