Sahih Al Bukhari Hadith Number 1908

Narrated Ibn `Umar: The Prophet (peace be upon him) said, “The month is like this and this,” (at the same time he showed the fingers of both his hands thrice) and left out one thumb on the third time.

30  Fasting
Hadith number 1908
Volume no 3
Chapter: 11 (Start observing Saum on seeing the crescent of Ramadan, and stop on seeing the crescent (of Shawwal).”