Rashi on Deuteronomy 10:16/רש"י על דברים י׳:ט״ז

1ערלת לבבכם [YE SHALL CIRCUMCISE] THE FORESKIN OF YOUR HEART — This means: ye shall remove the closure and cover that is on your hearts, [which prevent My words gaining entrance to them] (cf. Rashi on Exodus 6:12 and Leviticus 19:23).