Al-Allaamah Shaykh Saalih al-Fawzaan (may Allaah preserve him) stated:
"Likewise (from the beneficial matters of Hajj) is the Du'aa of 'Arafah. The Prophet (sallallaahu alayhi wa sallam) mentioned:
"The best Du'aa, is the Du'aa of 'Arafah. The best of that which myself and the Prophets before me have said is: -Laa Ilaaha Illallaah Wahdahu Laa Shareeka Lahu, Lahul Mulk Wa LahulHamd Wa Huwa Alaa Kulli Shay In Qadeer- None has the right to be worshiped except Allaah, who is alone without partners. For Him is the dominion and the praise. He is capable of doing all things."
(Musnad of Ahmad and Sunan of At-Tirmidhee Declared good by Shaykh Al-Albaani)
This is the best of that which the Prophet (sallallaahu alayhi wa sallam) said and He named it the Du'aa of 'Arafah. That is because the Du'aa is of two types:
1. Du'aa of worship. The statement ((Laa Ilaaha Illallaah)) is the Du'aa of worship and praise for Allaah. All of this is the Du'aa of worship.
2. Du'aa of request. It is that you ask Allaah to forgive you, to have mercy upon you, or that you ask that He provides for you, guides you, make you firmly established (upon the religion ofIslaam), forgive your parents, forgive your children and forgive your brothers... This is the Du'aa of request and it is that you ask Allaah (for that which you are seeking).
The example of this is the statement of Allaah:
"You (Allaah) alone we worship and You (Allaah) alone we seek for help"
So the statement "You (Allaah) alone we worship" this is the Du'aa of worship. The statement "You (Allaah) alone we seek for help", this is the Du'aa of request. Therefore, we have in one verse the Du'aa of worship and the Du'aa of request."
Source: Aqeedatul-Hajj Fee Daw al-Kitaab wa as-Sunnah pgs. 12-13 by Al-Allaamah Shaykh Saalih Al-Fawzaan (may Allaah preserve him)
Sisters Upon Al-Istiqaamah