Alḥamdulillāh, the day of ‘Arafah (ninth day of Dhū al-Ḥijjah) is upon us! It is from the Sunnah of our beloved Prophet (ﷺ) to say this supplication as much as you can on this blessed day:
The Prophet (ﷺ) said regarding this:
"The best supplication is the one made on the Day of ‘Arafah. The best thing ever said by me and the previous messengers is: There is no deity worthy of worship other than Allāh, who is alone, having no partner. His is the dominion, and to Him belongs all praise. He is, of everything, All Capable." (1)
And Allāh (ﷻ) knows best.
May Allāh bless you, and shukran.
Sources :
1. Collected in Jāmiʾ at-Tirmidhī 3585 and graded ḥasan by al-Albanī, though Imām at-Tirmidhī pointed out a weakness in it's chain. However, this ḥadīth has other supportng chains which allowed the scholars of aḥadīth to accept it's authenticity. (Bakkah.net)