Reference : Hisn al-Muslim 133

يَسْتَعِيذُ بِاللَّهِ.
يَنْتَهِي عَمَّا شك فِيهِ.
A`ūdhu billāh

(Say:) I seek refuge in Allah. 
(Then you should desist from doing what you are in doubt about.)

Reference:
Al-Bukhari, cf. Al-Asqalani, Fathul-Bari 6/336, Muslim 1/120.

Chapter: For one afflicted with doubt in his faith