((جَزَاكَ اللَّهُ خَيْراً)).
(If someone does you a favour and you say: (May Allah reward you with goodness.) then you have indeed excelled in praising him.)
At-Tirmidhi (no. 2035). See also Al-Albani, Sahih At-Tirmidhi 2/200 and Sahihul-Jami' (no. 6244).