Reference : Hisn al-Muslim 198

جَزاكَ اللهُ خَـيْراً
Jazākallāhu khayra.

May Allah reward you with good.

Reference:
At-Tirmidhi (no. 2035). See also Al-Albani, Sahih At-Tirmidhi 2/200 and 
Sahihul-Jami' (no. 6244).

Chapter: To one who does you a favour