Arabic text
عَنْ أَبِي قَتَادَةَ رضي الله عنه قَالَ: قَالَ النَّبِيُّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ: «الرُّؤْيَا الصَّالِحَةُ مِنَ اللَّهِ، وَالحُلُمُ مِنَ الشَّيْطَانِ، فَإِذَا حَلَمَ أَحَدُكُمْ حُلُمًا يَخَافُهُ فَلْيَبْصُقْ عَنْ يَسَارِهِ، وَلْيَتَعَوَّذْ بِاللَّهِ مِنْ شَرِّهَا، فَإِنَّهَا لاَ تَضُرُّهُ».
Translation
Abu Qatādah (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) said: "The good vision is from Allah, and the dream is from the devil. So, if any of you has a dream of which he is afraid, let him spit on his left and seek refuge with Allah from its evil, for it will not harm him."
Explanation
The Prophet (may Allah's peace and blessings be upon him) informed that the good and delightful vision is from Allah, whereas the dream, which is seeing what one dislikes and what saddens him, is from the devil. So, whoever sees what he dislikes should spit on his left and seek refuge with Allah from its evil, and it will not harm him as Allah has made what is mentioned a means of protection against the harm resulting from such a vision.
Source: HadeethEnc.com