Get The Full Reward Of Salaah
Rasoolullaah (صلى الله عليه وسلم) said, ‘Whoever recites the following three times after Salaat shall get the full reward of his Salaat (Tirmidhi)
سُبْحَانَ رَبِّكَ رَبِّ الْعِزَّةِ عَمَّا يَصِفُونَ
وَسَلَامٌ عَلَى الْمُرْسَلِينَ
وَالْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ
subHaana rabbika rabbil ‘Izzati ‘Ammaa yaSifoon
wa salaamun ‘Alal mursaleen
wal Hamdu lillaahi rabbil ‘AAlameen
037.180 Glory to thy Lord, the Lord of Honour and Power! (He is free) from what they ascribe (to Him)!
037.181 And Peace on the apostles!
037.182 And Praise to God, the Lord and Cherisher of the Worlds.
This reward is mentioned in a number of Ahadith (See Mujamul Kabeer Vol.5 Pg.211 , Majmauz Zawaid Vol.10 Pg.103, Masannaf Abdul Razzaaq Vol.2 Pg.236 , Tafsir ibn Abi Haatim Vol.10 Pg.3234), with some narrations mentioning its recitation once, not thrice.
It would be better to recite it after every salah, as mentioned in some narration, however it was the practice of Nabi (صلى الله عليه وسلم) to recite it once after the Fard salah. (Ibn Abi Shaibah Vol.1 Pg.269, Mujamul Kabeer Vol.11 Pg.115, Abu Ya’laa Vol.2 Pg.363)