بُلُوغُ الْمَرَامِ
Bulugh al-Maram 353
The Book of Prayer
وَلِمُسْلِمٍ : { كَانَ إِذَا طَلَعَ اَلْفَجْرُ لَا يُصَلِّي إِلَّا رَكْعَتَيْنِ خَفِيفَتَيْنِ } [1]
Muslim has: 'He never prayed after the break of dawn except two light Rak'at.'
Sunnah.com reference: Book 2, Hadith 256