Musnad Ahmad 1153:1961

Yazid ibn Harun narrated to us, he said: Asim al-Ahwal informed us, from Abu al-Walid, from Aisha, she said: The Prophet, peace and blessings of Allah be upon him, would not sit after his prayer 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."