Narrated Usama bin Zaid (RA):
Allah's Messenger (ﷺ) said. "If one is done a kindness and [expresses his gratitude] to his benefactor by saying 'May Allah give you a good reward', he has fully expressed his appreciation." [at-Tirmidhi reported it, and Ibn Hibban graded it Sahih (authentic)].
Reference: Bulugh al-Maram 1383
In-book reference: Book 13, Hadith 10
English translation: Book 13, Hadith 1383