Deuteronomy 24:1

This is that permission which the Pharisees erroneously referred to as a precept, Mat 19:7, Moses commanded to give a writing of divorcement. It was not so; our Saviour told them that he only suffered it because of the hardness of their hearts, lest, if they had not had liberty to divorce their wives, they should have ruled them with rigour, and it may be, have been the death of them. It is probable that divorces were in use before (they are taken for granted, Lev 21:14), and Moses thought it needful here to give some rules concerning them. 1. That a man might not divorce his wife unless he found some uncleanness in her, Deu 24:1. It was not sufficient to say that he did not like her, or that he liked another better, but he must show cause for his dislike; something that made her disagreeable and unpleasant to him, though it might not make her so to another. This uncleanness must mean something less than adultery; for, for that, she was to die; and less than the suspicion of it, for in that case he might give her the waters of jealousy; but it means either a light carriage, or a cross froward disposition, or some loathsome sore or disease; nay, some of the Jewish writers suppose that an offensive breath might be a just ground for divorce.

— Matthew Henry's Concise Commentary (public domain)