al-Bukhari.

[Commentary]
These ḥadiths deal with dreams, a subject we have already touched upon. We stated that dreams are of three kinds:

1. A good, righteous dream. This is from Allah, exalted is He. We previously noted that it brings delight and is an immediate glad tiding for the believer.

2. The ḥulm (nightmare). This is from Satan and, more often than not, contains what a person detests. Satan shows it to the sleeper in order to alarm him, spoil his mood, and plunge him into grief—he may even fall ill—because Satan is the enemy of humanity and rejoices in whatever harms or saddens people. Allah the Exalted says: “Secret counsels are only from Satan, that he may grieve those who believe, yet he cannot harm them in the least except by Allah’s permission. In Allah let the believers place their trust.” ⦗Quran 58:10⦘

The nightmare, then, is what a person sees in his sleep that he hates and finds disturbing. Yet, by Allah’s grace—Who has appointed a cure for every malady—the remedy for a nightmare is as follows:

First, the person should spit lightly to his left three times and seek refuge with Allah