once
جَزَاكَ اللَّهُ خَيْراً
Edition transliteration
If someone does you a favour and you say: (Jazakal-lahu khayran.) then you have indeed excelled in praising him.
Meaning
If someone does you a favour and you say: (May Allah reward you with goodness.) then you have indeed excelled in praising him.
Source and references
Hisn al-Muslim, entry 198.The reference is copied from the edition; separate editorial verification is not complete yet.
Source edition: Fortress of the Muslim. Open source ↗