Sahih Al Bukhari Hadith Number 1909

Narrated Abu Huraira: The Prophet (peace be upon him) or Abul-Qasim said, “Start fasting on seeing the crescent (of Ramadan), and give up fasting on seeing the crescent (of Shawwal), and if the sky is overcast (and you cannot see it), complete thirty days of Sha’ban.”

30  Fasting
Hadith number 1909
Volume no 3
Chapter: 11 (Start observing Saum on seeing the crescent of Ramadan, and stop on seeing the crescent (of Shawwal).”