Genesis 15:13

But note what is said to Abraham, "Know of a surety that your seed shall be a stranger in a land not theirs, and they shall reduce them to servitude, and shall afflict them four hundred years." This is most clearly a prophecy about the people of Israel, who were to be in servitude in Egypt. Not that this people was to be in that servitude under the oppressive Egyptians for four hundred years, but it is foretold that this should take place in the course of those four hundred years. It is written of Terah the father of Abraham, "And the days of Terah in Haran were 205 years," not because they were all spent there but because they were completed there. So it is said here also, "And they shall reduce them to servitude and shall afflict them four hundred years" … because that number was completed, not because it was all spent in that affliction. The years are said to be four hundred in round numbers, although they were a little more—whether you reckon from this time when these things were promised to Abraham, or from the birth of Isaac, as the seed of Abraham, of which these things are predicted. For, as we have already said above, from the seventy-fifth year of Abraham, when the first promise was made to him, down to the exodus of Israel from Egypt, there are reckoned 430 years, which the apostle thus mentions: "And this I say, that the covenant confirmed by God, the law, which was made 430 years after, cannot disannul, that it should make the promise of no effect." So then these 430 years might be called four hundred, because they are not much more, especially since part even of that number had already gone by when these things were shown and said to Abraham in vision, or when Isaac was born in his father's one hundredth year, twenty-five years after the first promise, when of these 430 years there now remained 405, which God was pleased to call four hundred. No one will doubt that the other things that follow in the prophetic words of God pertain to the people of Israel.