Musannaf Ibn Abi Shayba 17:1216

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Musa ibn Ubaydah, from Muhammad ibn Thabit, from Abu Hurayrah, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If a man says to his brother, 'May Allah reward you with good' (Jazak Allahu Khayran), then he has been eloquent in praise."