Allah, Exalted is He, mentioned the two examples – of fire and water – in Surah ar Rad (The Thunder), but this time when speaking of the believers; Allah says,
“He sends down from the sky rain, and valleys flow according to their measure, and the torrent carries away the rising foam. And from that (ore) which they heat in the fire, making adornments and utensils, is a foam like it. Thus Allah shows (the example of) truth and falsehood. As for the foam, it vanishes (being) passed away; but as for that which benefits the people, it remains on the earth. Thus does Allah set forth examples.”(Sarah Ar Rad: 17)
In this ayah, Allah compared the Revelation that was revealed by Him to the (spiritual) life of the hearts, and the hearing and sight faculties to water that brings life to the earth with plants; and compared the hearts to the valleys, since a large heart can accommodate great amount of knowledge just as a large valley can accommodate a lot of water. As for a small heart, it can only accommodate according to its size just like a small valley. Hearts bear guidance and knowledge according to their capacity in a similar manner to the flow of water when it comes in contact with earth and runs over it; it picks up scum and foam. Similarly if knowledge and guidance come in contact with the hearts, they agitate what is present in them in the form of lusts and doubts to uproot them and carry them off similar to how medicine works; when it is consumed it interacts with the body to rid it of harmful elements in order to ensure achieving absolute healing despite the taste of the bitterness it causes. This is the way in which Allah “shows (the example of) truth and falsehood.”
Then he mentions the example with the fire, saying “And from that (ore) which they heat, making adornments and utensils, is a foam like it.”; it is filth and dirt that is removed during the process of casting gold, silver, copper and iron – the fire removes it, differentiating it and separating it from the valuable metal that is of benefit, and these impurities are cast and discarded “it vanishes (being) passed away“. In the same way, lusts and doubt are tossed by a believer’s heart and discarded, just as the flowing water and the fire discard the foam, scum and impurities, and what remains in the bottom of the valley is pure water which people drink from and irrigate with and give to their livestock. And in the same way, pure and unadulterated faith settles in the centre of the heart and takes root, from which the person himself benefits and others also benefit from it. Whoever does not comprehend these two examples and does not reflect over them, then he is not from among those people (who have the qualities mentioned in the two examples); and Allah is the Granter of success.