Sheikh `Abd al-Rahmân al-`Ajlân, lecturer at the Grand Mosque in Mecca
Saying “al-Salâh khayr min al-nawm” after “Hayyâ `alâ al-falâh” in the morning adhân is sunnah.
The Prophet (peace be upon him) told the Companion Abu Mahzûrah: “In the morning prayer, say “al-Salâh khayr min al-nawm” twice.”
This practice is referred to as tathwîb.
Source: Islam Today