*Du'a for the Day of 'Arafah*
The best invocation is that of the Day of Arafat, and the best that anyone can say is what I and the Prophets before me have said:
*لاَ إِلَهَ إِلاَّ اللّهُ وَحْدَهُ لَا شَرِيكَ لَهُ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُوَ عَلَى كُلِّ شَيْءٍ قَدِيرٌ*
"None has the right to be worshiped but Allah alone, Who has no partner. His is the dominion and His is the praise, and He is Able to do all things"
At-Tirmizi.