"He is sleeping." He said: "As-salatu khairum minan-nawm, As-salatu khairum minan-nawm (The prayer is better than sleep, the prayer is better than sleep). These words were approved of in the Adhan for the Fajr, and that is how it remained.  

أَنَّهُ أَتَى النَّبِيَّ ﷺ يُؤْذِنُهُ بِصَلاَةِ الْفَجْرِ فَقِيلَ هُوَ نَائِمٌ فَقَالَ الصَّلاَةُ خَيْرٌ مِنَ النَّوْمِ الصَّلاَةُ خَيْرٌ مِنَ النَّوْمِ فَأُقِرَّتْ فِي تَأْذِينِ الْفَجْرِ فَثَبَتَ الأَمْرُ عَلَى ذَلِكَ  

[Machine] Bilal came to the Prophet ﷺ to call him for prayer once, but it was said that he was asleep. So, he called out, "Prayer is better than sleep!" and it was affirmed for the Fajr prayer.  

أَنَّ بِلَالًا أَتَى النَّبِيَّ ﷺ يُؤْذِنُهُ بِالصَّلَاةِ مَرَّةً فَقِيلَ إِنَّهُ نَائِمٌ فَنَادَى «الصَّلَاةُ خَيْرٌ مِنَ النَّوْمِ» فَأُقِرَّتْ فِي صَلَاةِ الْفَجْرِ  

[Machine] Saad used to give the call to prayer for the Messenger of Allah ﷺ . Hafs narrated that my family told me that Bilal came to the Messenger of Allah ﷺ to give the call to prayer for Fajr prayer, and they said that he was asleep. So, Bilal called out in a loud voice, "Prayer is better than sleep." So, it was confirmed for the Fajr prayer.  

أَنَّ سَعْدًا كَانَ يُؤَذِّنُ لِرَسُولِ اللهِ ﷺ قَالَ حَفْصٌ فَحَدَّثَنِي أَهْلِي أَنَّ بِلَالًا أَتَى رَسُولَ اللهِ ﷺ لَيُؤْذِنَهُ بِصَلَاةِ الْفَجْرِ فَقَالُوا إِنَّهُ نَائِمٌ فَنَادَى بِلَالٌ بِأَعْلَى صَوْتِهِ الصَّلَاةُ خَيْرٌ مِنَ النَّوْمِ فَأُقِرَّتْ فِي صَلَاةِ الْفَجْرِ  

"عن عائشة قالت: جاء بلال إلى النبي ﷺ يؤذنه بصلاة الصبح فوجده نائِمًا، فقال: الصلاة خير من النوم، فأقرت في صلاة الصبح".