Allahumma a’inni ‘ala zikrika

 (اللَّهُمَّ أَعِنِّي عَلَى ذِكْرِكَ)

 is the beginning of a powerful and beloved supplication (dua) taught by Prophet Muhammad (SAW) to his companion Mu’adh bin Jabal.

 
The full dua is:

اللَّهُمَّ أَعِنِّي عَلَى ذِكْرِكَ، وَشُكْرِكَ، وَحُسْنِ عِبَادَتِكَ

Allahumma a’inni ‘ala zikrika, wa shukrika, wa husni ‘ibadatika. 
"O Allah, help me in remembering You, in giving thanks to You, and in worshipping You in the best manner".