"In the Name of Allah, the Most Gracious, the Merciful. (This letter is) from Muhammad bin 'Abdullah, to Heraclius. "...O people of the Scripture (Jews and Christians): Come to a word that is just between us and you that we worship none but Allah..." (V.3:64)
In-book : Book 97, Hadith 166
USC-MSA web (English) : Vol. 1, Book 93, Hadith 631 (deprecated)