O Allah, Knower of the Unseen and the Seen
اللَّهُمَّ عَالِمَ الْغَيْبِ وَالشَّهَادَةِ، فَاطِرَ السَّمَاوَاتِ وَالْأَرْضِ، رَبَّ كُلِّ شَيْءٍ وَمَلِيكَهُ، أَشْهَدُ أَنْ لَا إِلَٰهَ إِلَّا أَنْتَ، أَعُوذُ بِكَ مِنْ شَرِّ نَفْسِي وَشَرِّ الشَّيْطَانِ وَشِرْكِهِ
Allahumma 'alimal-ghaybi wash-shahadah, fatiras-samawati wal-ard, rabba kulli shay'in wa malikah, ash-hadu an la ilaha illa anta, a'udhu bika min sharri nafsi wa sharrish-shaytani wa shirkih.
O Allah, Knower of the unseen and the seen, Originator of the heavens and the earth, Lord and Owner of everything, I bear witness that there is no god but You. I take refuge with You from the evil of my own soul and from the evil of Shaytan and the shirk he calls to.
What it means
Notice the order of the two evils it names: your own self comes first, then Shaytan. Most of what goes wrong in a day starts as something you wanted, and this dua is honest about that before it blames anything external. You address the One who sees both the hidden and the visible, so there is no point in pretending.
When and how
Say it once in the morning, once in the evening, and once before sleeping.
Its virtue
The Prophet ﷺ taught it to Abu Bakr as-Siddiq and told him to say it morning, evening and when going to bed, reported in Sunan Abi Dawud 5067 and Jami at-Tirmidhi 3392.
Source
Abu Dawud 5067; At-Tirmidhi 3392sahih