He called on the People of the Book to believe in Allah without associating partners with Him

while calling the people of the book to the true religion, the prophet (saas) told them not to set up any partners in worship with him, and invited them to agree with muslims on a mutual point of faith:

say, "people of the book! come to a proposition which is the same for us and you-that we should worship none but allah and not associate any partners with him and not take one another as lords besides allah." if they turn away, say, "bear witness that we are muslims." (surah al 'imran: 64)

say: "people of the book! do not go to extremes in your religion, asserting other than the truth, and do not follow the whims and desires of people who were misguided previously and have misguided many others, and are far from the right way." (surat al-ma'ida: 77)

the prophet (saas) sent muadh (ra) to the yemen, and told him to call on the people of the book to worship allah alone:

"you are going to people of a (divine) book. first of all invite them to worship allah (alone)…"66

truly it is revelation sent down by the lord of all the worlds.
(surat ash-shu'ara: 192)