Musnad Ahmad 32:1364

Abu Sa’id, the freed slave of Banu Hashim, narrated to us, saying: ‘Abdullah ibn al-Muthanna narrated to us, saying: I heard Thumamah ibn Anas mentioning that Anas, when he spoke, would repeat his speech three times. And he mentions that the Prophet (peace and blessings of Allah be upon him), when he spoke, would repeat his speech three times, and he would ask for permission three times. Abu Sa’id said: And he narrated to us after that with this hadith: "That the Prophet (peace and blessings of Allah be upon him) would ask for permission three times."