Musannaf Ibn Abi Shayba 3:708

Abu Mu'awiyah narrated to us, from 'Asim, from 'Abdullah ibn al-Harith, from 'Aishah, who said: When the Messenger of Allah (may Allah bless him and grant him peace) would say the taslim, he would not sit except for the duration of what it takes to say: "O Allah, You are Peace, and from You is peace. Blessed are You, O Possessor of Majesty and Honor."