ibn al Qayyim
…Let us return to the commentary on the hadith of Harith, where he Prophet speaks of what will rescue a servant from the Foe: “And enjoins upon you the fast. Verily, the similitude of that is a man carrying a sack-full of musk in a crowd of people, all of them marvelling at its fragrance—for the breath of someone lasting is more fragrant to God, Most High, than the scent of musk.”
The Prophet uses the image of someone carrying a sack-full of musk concealed from view, hidden under his garments, after the habit of those who carry musk. Fasting is, likewise, hidden from the eyes of men and unperceived by their senses.
The fasting person’s limbs fast from sins; his tongue fasts from lies, base language and false witness; his stomach fasts from food and drink; and his pudenda fast from union. If he speaks, he says nothing to violate his fast; and if he acts, he does nothing to spoil his fast. All his speech is salutary and wholesome, as are his deeds—like the fragrance one smells while sitting next to the bearer of musk. Anyone who sits with a fasting person benefits from his presence and is safe from false witness, lies, base language and wrongdoing. This is the fast prescribed by the Sacred Law, not simply abstinence from eating and drinking.
Hence, a sound hadith states: “When someone does not refrain from speaking falsely and the action that springs from it and from ignorance, God does not need him to refrain from food and drink.” And in hadith: “Some who fast obtain nothing from it but hunger and thirst.”
True fasting is when the limbs fast from sin and the stomach fasts from food and drink. As food and drink can break the fast or spoil it, so sins can cut off its reward and spoil its fruits, as if one had not fasted at all.