They have certainly disbelieved who say, "Allāh is the third of three."1 And there is no god except one God. And if they do not desist from what they are saying, there will surely afflict the disbelievers among them a painful punishment.
Translation: Saheeh International (English)
1 i.e., one part of three, referring to the Christian concept of trinity.
The Christians who say that ‘Allah is part of the Trinity: Father, Son and Holy Spirit’ have committed disbelief. Allah is far above such a statement. Allah is not many, but He is only One God Who has no partner. If they do not stop saying such things, a painful punishment will afflict them.