The Messenger of Allah (may Allah bless him and grant him peace) said,
“The best du’a is the du’a made on the day of Arafa, and the best thing that I have said, and likewise the Prophets before me, is: There is no god but Allah, alone, without partner.”
Lā ilāha ill-Allāhu waḥdahū lā sharīka lah.
لا إله إلا الله وحده لا شريك له
– Narrated by Imam Malik in the Muwatta, and quoted by shaykh Salih al-Ja’fari in his Friday Lesson.