The Best Thing To Say On The Day of 'Arafah.

وعن عبد الله بن عمرو بن العاص رضي الله عنهما أن النبي صلى الله عليه وسلم قال : ( خير الدعاء دعاء يوم عرفة ، وخير ما قلت أنا والنبيّون من قبلي : لا إله إلا الله وحده لا شريك له ، له الملك ، وله الحمد ، وهو على كل شيء قدير ) رواه الترمذي ( 3585 ) وحسَّنه الألباني في " صحيح الترغيب " -  1536   

On the authority of Abdullah bin 'Amr bin Al 'Aas - May Allaah be pleased with him and his father- that the Messenger of Allaah Salallahu alayhi wasallam said : " The best of Dua'aa is the Du'a of day of 'Arafah. And the best of what I and the Prophets before me said :

'Laa Ilaaha Ilallaah Wahdahu Laa Shareeka Lah, Lahul Mulk, Wa Lahul Hamd, Wa Huwa 'Ala kulli shay'in qadeer'

( There is no one worthy of worship except Allaah alone without associating any partners with him, to him alone belongs the dominion and to him alone belongs the praises and He is able to do everything )"

Sunan At Tirmidhi : 3585. Al Albaani declared this hadeeth to be Hasan in Saheeh At Targheeb : 1536.

وعن عبد الله بن عمرو بن العاص  قال كان أكثر دعائه يوم عرفة : لا إله إلا الله وحده لا شريك له , له الملك وله الحمد , بيده الخير , وهو على كل شيء قدير.

رواه إمام أحمد في مسنده وقال الشيخ شعيب الارناؤوط: حسن لغيره  في تحقيق على مسند الإمام أحمد  رقم: ٦٩٦١

On the Authority of Abdullah bin 'amr bin Al 'Aas, radia Allaahu 'anhu He said " The most dua of the Messenger of Allaah Salallahu alayhi wasallam on day of 'arafah 

'Laa Ilaaha Ilallaah Wahdahu Laa Shareeka Lah, Lahul Mulk, Wa Lahul Hamd, Biyadihil Khayr  Wa Huwa 'Ala kulli shay'in qadeer'

( There is no one worthy of worship except Allaah alone without associating any partners with him, to him alone belongs the dominion and to him alone belongs the praises, In His hands is the good and He is able to do everything )"

Narrated by Imaam Ahmad in His Musnad 6961. Shu'ayb Al Arnaa'oot declared this Hadeeth to be Hasan Lighayrihi in his tahqeeq of the Musnad of Imaam Ahmad.