Our Lord, Do Not Let Our Hearts Deviate
رَبَّنَا لَا تُزِغْ قُلُوبَنَا بَعْدَ إِذْ هَدَيْتَنَا وَهَبْ لَنَا مِنْ لَدُنْكَ رَحْمَةً إِنَّكَ أَنْتَ الْوَهَّابُ
Rabbana la tuzigh qulubana ba'da idh hadaytana wa hab lana min ladunka rahmah, innaka antal-wahhab.
Our Lord, do not let our hearts deviate after You have guided us, and grant us mercy from Yourself. You are the Giver of all gifts.
What it means
Guidance is described here as something that can be lost after it has been received, which is why the dua is in the hands of people who already believe. The word wahhab, the Giver, is chosen on purpose: guidance is a gift, not a wage you have earned and can count on.
When and how
Say it in sujud and in your daily duas, particularly when you feel your certainty weakening.
Its virtue
It is a verse of the Quran, quoted from those firmly grounded in knowledge, and was a frequent dua of the righteous before us.
Source
Quran 3:8
Quranic text from Tanzil.net, Creative Commons Attribution 3.0.