Wallahi is a somali version of saying «i swear to god» (Wallah) it’s a short version of «Wallahi billahi tallahi» Which has become more popular than the arabic way (Wallah) And is a common used Toronto slang.
Mom: Abdullah Did you pray duhur?
Abdullah: yea mom
Mom: say wallahi!
Abdullah: yeah wallahi I did
Mom: say wallahi billahi tallahi
Abdullah: wallahi billahi tallahi, I prayed duhur.