Rashi on Deuteronomy 8:2/רש"י על דברים ח׳:ב׳

1התשמר מצותו [THAT HE MIGHT KNOW WHAT WAS IN THY HEART] WHETHER THOU WOULDST KEEP HIS COMMANDMENTS — viz., the commandment that ye should not put Him to the test, and that you should not doubt Him (i.e. His ways).