Sahih Bukhari Volume 2, Book 26, Hadith Number 744.

Narated By ‘Amr bin Maimun : I saw ‘Umar, offering the Fajr (morning) prayer at Jam’; then he got up and said, “The pagans did not use to depart (from Jam’) till the sun had risen, and they used to say, ‘Let the sun shine on Thabir (a mountain).’ But the Prophet contradicted them and departed from Jam’ before sunrise.”