It is narrated from Sayyiduna Bilal (may Allah be pleased with him) that in the Fajr adhan he used to say: "Hayya 'ala khayr al-'amal (Come to the best of deeds)." So the Messenger of Allah (peace and blessings be upon him) ordered that in its place should be said: "As-salatu khayrun min an-nawm (Prayer is better than sleep)," and saying "Hayya 'ala khayr al-'amal" was left off.
Hadith Referenceالمعجم الكبير للطبراني / باب الباء / 1067