"If someone does you a favour then you say:

جَزَاكَ اللَّهُ خَيْراً

May Allah reward you with good.

Jazaakallaahu khayran.

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