Midrash for Deuteronomy 29:3

Our Rabbis were taught: Concerning the above-cited verse (Deut. 5, 26) O, that they had such a heart as this always to fear Me, Moses said to Israel: "Ye are ungrateful, my children; for at the time the Holy One, praised be He! said to you, O, that they had such a heart, etc., ye should have said, 'Thou, Lord, grant it to us.' Your ungratefulness is also marked from (Num. 21, 5) And our soul loathed (Ib. b) this light bread. Your ungratefulness is also marked from the passage (Gen. 3, 12) The woman whom Thou gavest to be with me, etc." Moses, however, hinted this to Israel only after the forty years in which he led them in the desert, as it is said (Deut. 29, 3) But the Lord hath not given you a heart to know. Said Raba: "From this it may be inferred that one cannot find the real mind of his master, until the lapse of forty years."