The Messenger of Allāh صَلَّى اللّٰهُ عَلَيْهِ وَسَلَّمَ said:
“The best supplication is the one made on the Day of ‘Arafah. The best supplication ever said by myself and the previous Prophets is:
لَا إِلَهَ إِلَّا اللهُ، وَحْدَهُ لَا شَرِيكَ لَهُ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ، وَهُوَ عَلَى كُلِّ شَيْءٍ قَدِيرٌ
‘There is no deity worthy of worship [in truth] other than Allāh, who is alone, having no partner or associate. His is the dominion, and to Him belongs all praise. He has power over all; everything, All-Capable.’”
Saheeh at-Tirmidhi 3585 | Shaykh al-Albāni رحمه الله | Hasan
Saheeh al-Jāmi' 3274 | Shaykh al-Albāni رحمه الله | Hasan