It was narrated that Bilal came to the Prophet to call him for the Fajr prayer, and was told:
"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.

حَدَّثَنَا عَمْرُو بْنُ رَافِعٍ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ الْمُبَارَكِ، عَنْ مَعْمَرٍ، عَنِ الزُّهْرِيِّ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، عَنْ بِلاَلٍ، 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 .