From Sahih Bukhari (Hadith 663):
Narrated Malik Ibn Buhaina:Allah's Messenger (ﷺ) passed by a man praying two rak`at after the Iqama (had been pronounced). When Allah's Messenger (ﷺ) completed the prayer, the people gathered around him (the Prophet) or that man and Allah's Messenger (ﷺ) said to him (protesting), Are there four rak`at in Fajr prayer? Are there four rak`at in Fajr prayer?
Reference: Bukhari 663