Sahih Bukhari Volume 1, Book 11, Hadith Number 632

Narated By 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?”