Sahih Bukhari | Volume 1 | Hadith Number 632

Narrated 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?"