Hadith: It is part of the Sunnah that when the Muezzin says in the Adhān for the Fajr prayer: "Come to success," he then says: "prayer is better than sleep."

عن أنس رضي الله عنه قال: من السنة إذا قال المؤذن في أذان الفجر: حيَّ على الفلاح، قال: الصلاة خير من النوم.
[صحيح] - [رواه ابن خزيمة والدارقطني والبيهقي]

Anas (may Allah be pleased with him) reported: It is part of the Sunnah that when the Muezzin says in the Adhān for the Fajr prayer: "Come to success," he then says: "prayer is better than sleep."
[Authentic hadith] - [Ibn Khuzaymah - Al-Bayhaqi - Narrated by Ad-Daraqutny]

Explanation

This Hadīth points out that the Adhān of Fajr prayer includes a particular phrase that is not said in the Adhān of other prayers; namely: “prayer is better than sleep.” The Muezzin should say it after the phrase: “Come to success.”