"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
"أَتَى بلالٌ النَّبىَّ ﷺ يُؤْذِنُهُ بالصَّلاةِ مَرَّةً، فَقِيلَ إِنَّهُ نَائِمٌ، فَنَادَى: الصَّلاةُ خَيْرٌ منَ النَوْم فَأُقِرَّتْ فِى صَلاةِ الفَجْرِ".