Ar-Ra'd 13:17

He sends down rain from the sky, causing the valleys to flow, each according to its capacity, and the torrent carries along rising foam, similar to the slag produced from metal that people melt in the fire to make ornaments or tools. This is how Allah compares truth to falsehood: as for the residue, it is then cast away, but that which benefits people remains on the earth[18]. This is how Allah sets forth comparisons.

[18] Truth is the water that stays and people benefit from it, whereas falsehood is the scum that soon dissolves and disappears; likewise is the slag of minerals.

He sends down from the sky, rain, and valleys flow according to their capacity, and the torrent carries a rising foam. And from that [ore] which they heat in the fire, desiring adornments and utensils, is a foam like it. Thus Allāh presents [the example of] truth and falsehood. As for the foam, it vanishes, [being] cast off; but as for that which benefits the people, it remains on the earth. Thus does Allāh present examples.

17. He sends down water (rain) from the sky, and the valleys flow according to their measure, but the flood bears away the foam that mounts up to the surface - and (also) from that (ore) which they heat in the fire in order to make ornaments or utensils, rises a foam like unto it, thus does Allâh (by parables) show forth truth and falsehood[3]. Then, as for the foam it passes away as scum upon the banks, while that which is for the good of mankind remains in the earth. Thus Allâh sets forth parables (for the truth and falsehood, i.e. Belief and disbelief).

[3] (V.13:17) See the footnote (A) of (V.9:119).

He sendeth down water from the sky, so that valleys flow according to their measure, and the flood beareth (on its surface) swelling foam - from that which they smelt in the fire in order to make ornaments and tools riseth a foam like unto it - thus Allah coineth (the similitude of) the true and the false. Then, as for the foam, it passeth away as scum upon the banks, while, as for that which is of use to mankind, it remaineth in the earth. Thus Allah coineth the similitudes.