Or HaChaim on Deuteronomy 9:4/אור החיים על דברים ט׳:ד׳

1 אל תאמר בלבבו, "Do not say in your heart, etc." We can appreciate that the Israelites were not to credit their own righteousness as the reason they dispossessed the inhabitants of the land of Canaan and therefore Moses had to warn them not to do so. However, why were they not to credit their possession of the land to the sins of the Canaanites when the Torah itself gives this as the reason in verse 5? Perhaps Moses was afraid that the Israelites would think that they were only forbidden to take exclusive credit for conquering the land by ascribing it to their righteousness. However, as long as they would acknowledge that both their own righteousness coupled with the wickedness of the local inhabitants had led to those inhabitants being expelled or killed, this would be acceptable, Moses said that no thought of their own righteousness was acceptable under any circumstances.