Assalamu alaikum.
The prophet (SAW) said; “The best du’a done is the one during ARAFA, and the du’a I and the prophets before used to say is; Laa Ilaaha illaLlahu wahdahu laa sharikalahu Lahul mulku wa Lahul hamdu waHuwa alaa kulli shay’in qadeer”.
Translation:
“There’s no God worthy of worship except Allah, He is the One and Only, no partner or associate does He have, to Him belongs the entire kingdom, to Him belongs all gratitude and praise, and He is able to do anything”.
May Allah accept it from us and you.