Rashi on Deuteronomy 1:11/רש"י על דברים א׳:י״א

1יסף עליכם ככם אלף פעמים MAY HE ADD TO YOU A THOUSAND TIMES AS MANY MORE AS YE ARE — What is the force of saying further: ויברך אתכם כאשר דבר לכם AND MAY HE BLESS YOU EVEN AS HE HATH SPOKEN CONCERNING YOU? But the explanation is: They said to him, “Moses, you are setting a limit to our blessings (only a thousand times)! The Holy One, blessed be He, has already made a boundless promise to Abraham (Genesis 13:16) “… if one can count [the dust of the earth, then can thy seed also be counted]"! Moses replied to them: This (a thousand times) is from me (it is my blessing); but may He bless you even as He hath spoken concerning you!" (Sifrei Devarim 11).