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