Sahih Muslim Book 6, Hadith Number 2385.

Jabir (Allah be pleased with her) narrated that the Holy Prophet (may peace be upon him) separated himself from his wives for a month. (His wives said:) He came to us on the twenty-ninth day, whereupon we said: It is the twenty-ninth (day) today. Thereupon he said: So far as the month is concerned, (and he, with a view to explaining it) flapped his hands thrice, but held back one finger at the last turn.