What dua do I say when entering the bathroom?

Before entering

Anas reported that whenever the Prophet ﷺ went to answer the call of nature, he would say: "Allah-umma inni a'udhu bika minal khubuthi wal khaba'ith", meaning "O Allah, I seek Refuge with You from all offensive and wicked things (evil deeds and evil spirits)" (Sahih al-Bukhari 142).

Say it just before you step in, not once you're inside. If you're outdoors, for example camping or hiking, say it before you start.

Many scholars also recommend saying "Bismillah" first. This comes from a narration in Jami' at-Tirmidhi that some scholars consider weak, so it's a popular practice rather than a firmly established one. Saying it is fine, and so is leaving it.

After leaving

Aishah reported: "When the Prophet would exit the toilet he would say: 'Ghufranak'" (Jami' at-Tirmidhi 7), meaning "Grant me Thy forgiveness" (Sunan Abi Dawud 30). Scholars mention several reasons for asking forgiveness here, including that it's a moment when a person couldn't remember Allah aloud, and as thanks that Allah made the body healthy enough to get rid of what harms it.

Other bathroom etiquette

• No dhikr inside: most scholars say not to say Allah's name or recite Quran while relieving yourself. See Can I do dhikr in the bathroom?
• Left foot in, right foot out: scholars recommend this as good manners, in line with the habit of the Prophet ﷺ of starting good things with the right.
• Clean yourself properly: water is preferred, and toilet paper also counts. See Do Muslims have to use water instead of toilet paper?
• Cover up and keep private: modesty applies here too.

Modern bathrooms

Most Western homes have the toilet, shower and sink in one room. Since the dua is linked to relieving yourself, say it before entering when you're going to use the toilet. If you're only going in to wash your hands or brush your teeth, many people don't say it. If you forget the dua, it's not a sin; just remember next time.