What dua do I say when I lose something?

Start with the words of the Quran

Losing your keys, wallet or phone is a small musibah (misfortune), and the Quran praises those who respond to misfortune with these words:

The Prophet ﷺ taught that whoever is struck by a misfortune and says "Inna lillahi wa inna ilayhi raji'un, Allahumma'jurni fi musibati wakhluf li khayran minha" ("O Allah, reward me in my affliction and replace it with something better") will be given something better (Sahih Muslim 918). Many scholars mention it for small losses too. See What do I say when I hear bad news as a Muslim?

Then make your own dua

Allah knows exactly where your lost item is. Ask Him plainly: "O Allah, You know where it is, guide me to it." You can make dua in English or any language. See Can I make dua in my own words?

Some Muslims also recite "Rabbana innaka jami'un-nasi li yawmin la rayba fih" ("Our Lord, You will gather the people for a Day about which there is no doubt", Quran 3:9), asking the One who gathers all people to gather their lost things. This is a practice of some scholars, not a hadith, so treat it as a personal dua rather than a sunnah.

"La hawla wa la quwwata illa billah" is also perfect when you feel helpless. See What does "La hawla wa la quwwata illa billah" mean?

What if I find something someone else lost?

• Islam asks you to look after found property and make a real effort to return it, for example by handing it to the shop, the police or the lost-and-found office.
• Don't keep valuables just because the owner isn't around.
• Don't announce lost items loudly inside the mosque. The Prophet ﷺ disliked it and said the mosque wasn't built for that (Sahih Muslim 568). Tell the mosque office or post a notice outside instead.

Keep calm and accept the outcome

Sometimes the item turns up in five minutes; sometimes it's gone for good. Either way, staying patient earns reward, and nothing is truly lost with Allah. Losing things is also a chance to check how attached we've become to possessions.