Reference : Hisn al-Muslim 59

اللّهُـمَّ أَعِـنِّي عَلـى ذِكْـرِكَ وَشُكْـرِك ، 
وَحُسْـنِ عِبـادَتِـك
Allāhumma ‘a`innī `alā dhikrika, wa shukrika, 
wa ḥusni `ibādatik.


O Allah, help me to remember You, to give You thanks, 
and to perform Your worship in the best manner.

Reference:
Abu Dawud 2/86, An-Nasa'i 3/53. See also Al-Albani Sahih Abu Dawud 1 /284.

Chapter: After the last tashahhud and before salam