بُلُوغُ الْمَرَامِ
Bulugh al-Maram 179
The Book of Prayer
وَلِابْنِ خُزَيْمَةَ: عَنْ أَنَسٍ قَالَ: { مِنْ اَلسُّنَّةِ إِذَا قَالَ اَلْمُؤَذِّنُ فِي اَلْفَجْرِ: حَيٌّ عَلَى اَلْفَلَاحِ, قَالَ: اَلصَّلَاةُ خَيْرٌ مِنَ اَلنَّوْمِ } [1] .
Ibn Khuzaimah reported from Anas (RA): "It is from the Sunnah that when the Mu'adhadhin says [in the first Adhan of] Fajr (prayer time): 'Haiya 'alal-Falah (Come to the success)', he says (also) 'as-Salatu khairun minan-nawm (Prayer is better than sleep).'"
Sunnah.com reference: Book 2, Hadith 34