Genesis 13:16 in the World English Bible
I will make your offspring as the dust of the earth, so that if a man can count the dust of the earth, then your offspring may also be counted.
Context
15 For all the land which thou seest, to thee will I give it, and to thy seed for ever.
16 And I will make thy seed as the dust of the earth: so that if a man can number the dust of the earth, then shall thy seed also be numbered.
17 Arise, walk through the land in the length of it and in the breadth of it; for I will give it unto thee.