(25) باب اللِّعَانِ
Narrated Ibn `Umar:
The Prophet (holding out his ten fingers thrice), said, "The month is thus and thus and thus," namely thirty days. Then (holding out his ten fingers twice and then nine fingers), he said, "It may be thus and thus and thus," namely twenty nine days. He meant once thirty days and once twenty nine days.
Reference: Sahih al-Bukhari 5302
In-book reference: Book 68, Hadith 51
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 7, Book 63, Hadith 222