Narrated Abu Qatada Al-Ansari:

(a companion of the Prophet and one of his cavalry men) "I heard Allah's Apostle saying, "A good dream is from Allah, and a bad dream is from Satan;
so, if anyone of you had a bad dream which he disliked, then he should spit on his left and seek refuge with Allah from it, for it will not harm him."

حَدَّثَنَا يَحْيَى بْنُ بُكَيْرٍ، حَدَّثَنَا اللَّيْثُ، عَنْ عُقَيْلٍ، عَنِ ابْنِ شِهَابٍ، عَنْ أَبِي سَلَمَةَ، أَنَّ أَبَا قَتَادَةَ الأَنْصَارِيّ َ ـ وَكَانَ مِنْ أَصْحَابِ النَّبِيِّ ﷺ وَفُرْسَانِهِ قَالَ سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ " الرُّؤْيَا مِنَ اللَّهِ، وَالْحُلْمُ مِنَ الشَّيْطَانِ، فَإِذَا حَلَمَ أَحَدُكُمُ الْحُلُمَ يَكْرَهُهُ فَلْيَبْصُقْ عَنْ يَسَارِهِ وَلْيَسْتَعِذْ بِاللَّهِ مِنْهُ، فَلَنْ يَضُرَّهُ ".