"The Messenger of Allah commanded me (with Tathwib) in the Adhan for Fajr, and he forbade me to do so in the Adhan for 'Isha'."
Sunan Ibn Majah Book 3, Hadith 10
← Previous Hadith "The last instruction that the Messenger of Allah gave to me was that I should not appoint a Mu'adh-dhin who took payment for the Adhan." (sahih)

Sunan Ibn Majah Book 3, Hadith 9 Next Hadith → "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.

Sunan Ibn Majah Book 3, Hadith 11