Asim al-Ahwal عاصم الأحول · The Book on Salat (Prayer)

"When Allah's Messenger said the Salam he would not remain seated except long enough to say: (Allahumma antas-salam, wa minkas-salam, tabarakta dhal jalali wal-Ikram) 'O Allah! You are the One free of defects, and perfection is from You. Blessed are You, Possessor of Majesty and Honor.'"

(Another chain) which is similar, but he said: (Tabarakta ya dhal-jalali wal-ikram) "Blessed are You, O Possessor of Majesty and Honor."