Authentic Supplication Number 112.
It is reported of Mu’ath bin Jabal that he said that the Messenger of Allah took him by the hand and said “O’ Mu’ath I do by Allah love you, so make sure you never give up saying ‘Allahumma ‘a`innee `ala thikrika washukrika, wa husni `ibadatika (136) after every salat.
(136) O’ my ‘ilah help me in establishing your remembrance, thanking You, and worshipping You properly.
Reported by Abu Dawood, and An-Nisa’ee; and Al-Albani said its ascription is sound, and that all the men in its chain of ascription were trustworthy. He included this hadeeth in The Authentic of Good Sayings as #95.