Musannaf Ibn Abi Shayba 2:230

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