What dua do I say when I'm tempted to sin?

First: seek refuge in Allah

The Quran tells you exactly what to do the moment a bad urge hits:

So say "A'udhu billahi minash-shaytanir-rajim", out loud or quietly. It's short enough to say in any situation: on your phone at night, at a party, or at work.

Duas to keep your heart steady

• "Ya Muqallibal-qulub, thabbit qalbi 'ala dinik." The Prophet ﷺ said this dua often. When asked why, he explained that every heart is between the fingers of the Most Merciful, who turns it as He wills (Jami' at-Tirmidhi 2140, graded hasan).
• "Allahumma inni as'alukal-huda wat-tuqa wal-'afafa wal-ghina." "O Allah, I ask You for guidance, piety, chastity and contentment" (Sahih Muslim 2721). It's a perfect dua against temptations around desire, money and status.
• Prophet Yusuf's dua. When he was pressured to commit sin, he said: "My Lord, prison is more dear to me than that to which they invite me. And if You do not avert from me their plan, I might incline toward them" (Quran 12:33). He didn't trust his own strength; he asked Allah to turn the temptation away.

Walking away is rewarded

Feeling tempted isn't a sin. Acting on it is. The Prophet ﷺ said that if someone intends a bad deed and then doesn't do it, Allah records it as a full good deed (Sahih al-Bukhari 6491). Among the seven people shaded by Allah on the Day of Judgment is a man invited to sin by an attractive woman of status who says, "I fear Allah" (Sahih al-Bukhari 660). Every time you resist, you're earning something.

Unwanted thoughts that pop into your head aren't held against you either. See Will Allah punish me for my intrusive thoughts?

Practical steps that help the dua

• Change your place. Leave the room, put the phone in another room, or go out for a walk.
• Make wudu or pray two rak'ahs. It breaks the moment and calms you.
• Cut the triggers. Unfollow accounts, block sites, avoid certain places. See How do I deal with social media addiction as a Muslim?
• If you slip, repent quickly. Don't let one mistake turn into giving up. See Will Allah forgive me if I keep committing the same sin?