I am looking for a translation of a Swahili proverb printed on a piece
of fabric I recently purchased in Tanzania.  It reads: "sitomsahau
panya kwa kitendo alichonifanya"

Thank you.

Proverb translation:

Does not a rat forget what it did?

Sitomsahau (verb): (am not prefix (sitom) + forget (sahau))
Panya (noun): rat
Kwa (prepostion): from/of
Kitendo (noun): act, action, practice
Alichonifanya (verb): 3rd person singular of to do/to make

Hope this helps!

Sitomsahau Panya Kwa Kitendo Alichonifanya
I won't forget rat (a rat) for what he did to me.  

Though we don't know exactly what the "rat" did but from the title (in
swahili) its sounds, its usually means what the rat did was something
not so good (negative)