Abu Qatada reported Allah's Messenger (ﷺ) as saying:

A good vision is from Allah and a bad dream (hulm) is from the satan; so if one of you sees anything (in a dream) which he dislikes, he should spit on his left side thrice and seek refuge with Allah from its evil, and then it will never harm him. Abu Salama said: I used to see dreams weighing more heavily upon me than a mountain; but since I heard this hadith I don't care for it (its burden).

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ بْنِ قَعْنَبٍ، حَدَّثَنَا سُلَيْمَانُ، - يَعْنِي ابْنَ بِلاَلٍ - عَنْ يَحْيَى بْنِ سَعِيدٍ، قَالَ سَمِعْتُ أَبَا سَلَمَةَ بْنَ عَبْدِ الرَّحْمَنِ، يَقُولُ سَمِعْتُ أَبَا قَتَادَةَ، يَقُولُ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ ‏

‏ الرُّؤْيَا مِنَ اللَّهِ وَالْحُلْمُ مِنَ الشَّيْطَانِ فَإِذَا رَأَى أَحَدُكُمْ شَيْئًا يَكْرَهُهُ فَلْيَنْفِثْ عَنْ يَسَارِهِ ثَلاَثَ مَرَّاتٍ وَلْيَتَعَوَّذْ بِاللَّهِ مِنْ شَرِّهَا فَإِنَّهَا لَنْ تَضُرَّهُ ‏

‏ ‏.‏ فَقَالَ إِنْ كُنْتُ لأَرَى الرُّؤْيَا أَثْقَلَ عَلَىَّ مِنْ جَبَلٍ فَمَا هُوَ إِلاَّ أَنْ سَمِعْتُ بِهَذَا الْحَدِيثِ فَمَا أُبَالِيهَا ‏.‏