Musannaf Ibn Abi Shayba 2:231

Abu Bakr narrated to us, he said: Waki’ narrated to us, from ‘Ubayd Allah ibn al-Walid, from Muhammad ibn Nafi’, from ‘Aisha, she said: I do not see this verse having been revealed except regarding the callers to prayer (mu’adhdhins): "And who is better in speech than one who invites to Allah and does righteousness and says, 'Indeed, I am of the Muslims?'"