Bulugh al-Maram 2 The Book of Prayer كتاب الصلاة 34

وَلِابْنِ خُزَيْمَةَ: عَنْ أَنَسٍ قَالَ: { مِنْ اَلسُّنَّةِ إِذَا قَالَ اَلْمُؤَذِّنُ فِي اَلْفَجْرِ: حَيٌّ عَلَى اَلْفَلَاحِ, قَالَ: اَلصَّلَاةُ خَيْرٌ مِنَ اَلنَّوْمِ } 1 . 1 – رواه ابن خزيمة (386) بسند صحيح.

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).’”