Sahih Muslim Book 6, Hadith Number 2389.

Chapter : The month may consist of twenty-nine days.

Sa’d b. Abi Waqqas (Allah be pleased with him) said that the Messenger of Allah (may peace be upon him) struck his hand against the other and (then with the gesture of his two hands) said: The month is thus, thus (two times). He then withdrew (one of) his fingers at the third turn.