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)]. (ID)
Reference (ID): Bulugh al-Maram 1383 (ID)
In-book reference (ID): Book 13, Hadith 10 (ID)
English translation (ID): Book 13, Hadith 1383 (ID)