Midrash su Amos 5:3

R. Yehoshua b. Karchah says: (Koheleth 1:4) "A generation goes and a generation comes, and the earth stands forever.": What was created for what? The earth (was created) for the generation (of men). It should be written, thus: "The earth goes and the earth comes, and the generation stands forever!" But because they changed (i.e., corrupted) their ways, the Holy One Blessed be He changed because of them the ordinance of creation. And thus is it written (Hoshea 2:1) "And the number of the children of Israel shall be as the sand of the sea, which cannot be measured and which cannot be numbered!" (The resolution:) When Israel do the will of the L-rd, they are as the sand of the sea, which cannot be measured and which cannot be numbered. And if not, then the number of the children of Israel shall be." And it is written (Isaiah 30:17) "… until you will be left as a beacon on the top of a mountain," and (Amos 5:3) "For thus said the L-rd G-d: The city that went out with a thousand shall leave a hundred, and that which went out with a hundred shall leave ten, to the house of Israel." Variantly: "And the number of the children of Israel shall be" — This is the number of Heaven (i.e., Only G-d can give the exact number.) "as the sand of the sea, etc." — This is the number of man, (who is not capable of counting them).