Rashi on Deuteronomy 22:7/רש"י על דברים כ״ב:ז׳

1למען ייטב לך וגו׳ THAT IT MAY BE WELL WITH THEE etc. — If in the case of an easy command which involves no monetary loss, Scripture states “Do this in order that it may be well with thee and thou mayest prolong thy days”, it follows à fortiori that this at least will be the grant of the reward for the fulfilment of commands which are more difficult to observe (Chullin 142a).