The Prophet sal Allaahu Alayhi wa sallam said:
“The best of du’aa’ is du’aa’ on the day of ‘Arafah, and the best that I and the Prophets before me said is
لَا إِلَهَ إِلَّا اللهُ ، وَحْدَهُ لَا شَرِيكَ لَهُ ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ ، وهُوَ عَلَى كُلِّ شَيْءٍ قَدِيرٌ
‘Laa ilaaha ill-Allaah wahdahu la shareeka lah, lahu’l-mulk wa lahu’l-hamd wa huwa ‘ala kulli shay’in qadeer”
“There is no god but Allaah alone, with no partner or associate; His is the dominion, to Him be praise, and He has power over all things.”
(al-Tirmidhi-hasan by al-Albaani)
Of course also do a lot of Istighfaar, other adhkaar and send salutations on the Prophet sal Allaahu Alayhi wa sallam.
And Allaah knows best