Musnad Ahmad 1153:329

Waki’ narrated to us, from Sufyan, from ‘Asim ibn Sulayman, from ‘Abdullah ibn al-Harith, from ‘Aishah, that the Prophet, peace and blessings of Allah be upon him, used to say when he performed the taslim: "O Allah, You are As-Salam (the Perfection/the Peace), and from You is As-Salam (peace/security). Blessed are You, O Possessor of Majesty and Honor."