Musannaf Ibn Abi Shaybah

Hazrat Abu Huraira (may Allah be pleased with him) narrates that the Holy Prophet (peace and blessings be upon him) wrote a letter to a person from among the People of the Book, in which he wrote: "Accept Islam, you will be safe." The Prophet (peace and blessings be upon him) had not yet finished writing the letter when a letter from that person arrived. It was read out, and it contained greetings (salaam). So the Holy Prophet (peace and blessings be upon him) also replied to the greeting at the end of his letter.