Sahih Muslim Book 6, Hadith Number 2386.

Abu Zubair is reported to have heard Jabir b. ‘Abdullah (Allah be pleased with both of them) as saying: The Apostle of Allah (may peace be upon him) separated himself from his wives for a month. (His wives said:) He came to us on the morning of the twenty-ninth. Upon this some, of the people said: It is the morning of twenty-ninth (according to our calculation). Upon this the Apostle of Allah (may peace be upon him) said: The month may also consist of twenty-nine days. The Apostle of Allah (may peace be upon him) then flapped his bands thrice, twice with all the fingers of both his hand (to indicate twenty-nine) and by the third time with nine (fingers).