Musannaf Ibn Abi Shayba 2:229

Abu Bakr (Waki') narrated to us, he said: Yazid and Waki' narrated to us, they said: Isma'il narrated to us, from Shubayl ibn 'Awf, he said: 'Umar said: "Who are your mu'adhdhins (callers to prayer)?" They said: "Our slaves and our freedmen." He said: "That is indeed a great deficiency for you." Except that Waki' said: "A lot" or "a great" (deficiency).