Unasafiri.

The verb is kusafiri "to travel" and the noun is safari "a trip, journey".

These are loanwords from Arabic and Arabic has a pretty complicated system of deriving words from each other not by just adding prefixes or suffixes but by changing (or adding/subtracting) the vowels between three consonants, as well as using prefixes and suffixes. You'll see a lot of Arabic loanwords in Swahili that have different vowels. This is a good example. Here's another.

wizara = ministry
waziri = minister

And quite a big family of words comes from the Arabic root k-t-b which has to do with writing. Here are some examples.

kitabu = book (the Arabic is kitab, with the plural being kutub, but in Swahili, the plural is formed the Bantu way: vitabu)
maktaba = library
katiba = text; constitution; declaration
katibu = clerk, secretary
mkataba = contract

So, basically, sometimes you might notice funny changes in the vowels in related words and if that happens, you're probably dealing with loans from Arabic.