Surah an-Nas
قُلْ أَعُوذُ بِرَبِّ النَّاسِ، مَلِكِ النَّاسِ، إِلَٰهِ النَّاسِ، مِنْ شَرِّ الْوَسْوَاسِ الْخَنَّاسِ، الَّذِي يُوَسْوِسُ فِي صُدُورِ النَّاسِ، مِنَ الْجِنَّةِ وَالنَّاسِ
Qul a'udhu bi-rabbin-nas. Malikin-nas. Ilahin-nas. Min sharril-waswasil-khannas. Alladhi yuwaswisu fi sudurin-nas. Minal-jinnati wan-nas.
Say: I take refuge with the Lord of mankind, the King of mankind, the God of mankind, from the evil of the sneaking whisperer, who whispers into the hearts of mankind, from among the jinn and mankind.
What it means
Where al-Falaq deals with harm from outside, this surah deals with harm that gets inside: the whisper that talks you into despair, suspicion or sin. It is called the sneaking whisperer because it withdraws the moment you remember Allah, which is exactly what reciting this does.
When and how
Recite three times each morning and evening, after every prayer, and before sleep. It is also excellent to say quietly whenever an intrusive thought will not leave.
Its virtue
The Prophet ﷺ said no one has ever sought refuge with anything like these two surahs, reported in Sahih Muslim 814.
Source
Quran 114
Quranic text from Tanzil.net, Creative Commons Attribution 3.0.