Why is "Assalatu Khairum Minan Naum" only said in the fajr/morning azan/call for prayer?

The phrase "Assalatu Khairum Minan Naum," which translates to "Prayer is better than sleep," is specifically included in the Fajr (morning) call to prayer (adhan) to emphasize the importance of the morning prayer and its unique significance. This practice is supported by a hadith in which the Prophet Muhammad ﷺ highlighted the value of the Fajr prayer.

In Sahih Muslim, it is narrated that the Prophet ﷺ said, "Prayer in congregation is superior to praying alone by twenty-five degrees." He added, "The angels of the night and the angels of the day gather together in the Fajr prayer." Abu Huraira said, Recite if you will: 'And the recitation of dawn is ever witnessed' Sahih Muslim 5:306SunniSAHIH. This narration underscores the special status of the Fajr prayer, as it is attended by angels from both night and day, making it a significant moment for worship.

Additionally, Islamic scholars explain that this phrase serves as a reminder to the faithful about the importance of prioritizing prayer, especially at a time when many may still be inclined to sleep. The Fajr prayer marks the beginning of the day, and awakening for it establishes a routine of devotion that sets a positive tone for the rest of the day.