Call to Prayers (Adhaan)

Bukhari :: Book 1 :: Volume 11 :: Hadith 632

Narrated Malik Ibn Buhaina:

Allah's Apostle passed by a man praying two Rakat after the Iqama (had been pronounced). When Allah's Apostle completed the prayer, the people gathered around him (the Prophet) or that man and Allah's Apostle said to him (protesting), Are there four Rakat in Fajr prayer? Are there four Rakat in Fajr prayer?"