Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.098.

It is part of the shari’ah that the caller to prayer say, “as-salaatu khairun min an-naum (prayer is better than sleep) in the morning azhan. Abu Mahzhurah asked the Prophet, upon whom be peace, to teach him the azhan, and he told him, “If it is the morning azhan, say, as-salaatu khairun min an-naum, as-salaatu khariun min annaum. Allahu akbar, Allahu akbar. La illaha illal-lah.” (Related by Ahmad and Abu Dawud.) It is to be said only in the morning azhan.