Du'a when entering Madinah

اللَّهُمَّ هَذَا حَرَمُ نَبِيِّكَ، فَاجْعَلْهُ وِقَايَةً لِي مِنَ النَّارِ، وَآمِنَّا مِنَ الْعَذَابِ وَسُوءِ الْحِسَابِ

Allahumma hadha haramu nabiyyika, faj'alhu wiqayatan li minan-nari, wa aminna minal-'adhabi wa su'il-hisab

O Allah, this is the sanctuary of Your Prophet, so make it a protection for me from the Fire, and grant us security from the punishment and a bad reckoning.

A supplication from the fiqh manuals for entering Madinah; not a narration from the Prophet (peace be upon him).