Musnad Ahmad 32:1845

Mu'awiyah ibn 'Amr narrated to us, Za'idah narrated to us, from al-A'mash, who said: It was narrated to me from Anas ibn Malik, from the Prophet, may Allah bless him and grant him peace, who said: "The people with the longest necks on the Day of Resurrection will be the mu'adhdhins (those who call the adhan)."