Sahih Adab al-Mufrad

851/1112 (Hasan) from Uqbah ibn Amir al-Juhani: That he passed by a man whose appearance was that of a Muslim, so he greeted him with salam, and the man replied: "And upon you be peace, and the mercy of Allah, and His blessings." Then a boy said to him: "He is a Christian!" So Uqbah got up and followed him until he caught up with him. He said: "Indeed, the mercy of Allah and His blessings are upon the believers, but may Allah prolong your life, and increase your wealth and your children."(1)