Allahumma a`inni ala zikrika, wa shukrika wa husni ibadatika.

Narrated by Mu`adh ibn Jabal r.a.:

The Messenger of Allah (Peace Be Upon Him) took hold of my hand and said, "O Mu`adh! By Allah I love you.”  Mu’adh responded, “May my father and mother be sacrificed for you, O Messenger of Allah (saw); I love you too.”

Then the Prophet (saw) said, ”I advise you, o Mu’adh to never forget to recite after every prayer “Allahumma a`inni ala zikrika, wa shukrika wa husni ibadatika.”  (O Allah help me remember you, to be grateful to you and to worship you in the most excellent manner.)

Related by Ahmad, Abu Dawud, an-Nasa'i, Ibn Khuzaimah, Ibn Hibban and al-Hakim