From ‘Amr bin Shu’aib who said from his father who said from his grandfather that the Prophet (sallAllaahu ‘alayhi wa sallam) said:
"The best supplication is the supplication of the day of ‘Arafah; and the best of that which I and the Prophets before me said is:
لَا إِلَهَ إِلَّا اللهُ، وَحْدَهُ لَا شَرِيكَ لَهُ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ، وَهُوَ عَلَى كُلِّ شَيْءٍ قَدِيرٌ
Laa ilaaha illallaahu wahdahu laa shareeka lahu, lahul mulku wa lahul hamdu wa huwa ‘alaa kulli shay-in qadeer.
‘There is nothing worthy of worship except Allaah alone – without any partners for Him. Unto Him is the kingdom and for Him are all praises; and He has ability over all things’”
[Reported by At-Tirmidhee and was declared Hasan (sound) by Al-‘Allaamah Al-Albaanee in his Saheeh Sunan At-Tirmidhee (no.3585) (see also: As-Saheehah no.1503 & Saheeh Al- Jaami’ As-Sagheer no. 3274]