O Turner of Hearts
يَا مُقَلِّبَ الْقُلُوبِ ثَبِّتْ قَلْبِي عَلَىٰ دِينِكَ
Ya muqallibal-qulub, thabbit qalbi 'ala dinik.
O Turner of hearts, make my heart firm upon Your religion.
What it means
The Prophet ﷺ said this often, and when he was asked whether he feared for himself he explained that hearts are between two of the fingers of the Most Merciful, turned as He wills. If the best of creation asked for his heart to be held steady, nobody should assume their own faith will simply stay put.
When and how
Say it often, especially during periods of doubt, temptation or spiritual flatness.
Its virtue
Reported from the Prophet ﷺ as the dua he said most frequently, in Jami at-Tirmidhi 3522.
Source
At-Tirmidhi 3522hasan