بُلُوغُ الْمَرَامِ
Bulugh al-Maram 356
The Book of Prayer
وَلِمُسْلِمٍ : { رَكْعَتَا اَلْفَجْرِ خَيْرٌ مِنْ اَلدُّنْيَا وَمَا فِيهَا } [1]
Muslim has: "The two Rak'at at dawn are better than this world and what it contains."
Sunnah.com reference: Book 2, Hadith 259