How do you say God be with you in Swahili?

upaji wa Mungu

Upaji (from the verb kupa, to give) is an abstract noun for giving, generosity, etc., and in religious texts may mean the gift of God. But the standard Word for gift is zawadi, and most Swahili-speakers (everyone this writer has heard) would use this Word, zawadi ya Mungu. It is the basic meaning behind the naming of a child zawadi: the child is a gift from God.