Pray as though everything depended on God. Work as though everything depended on you.

Augustine of Hippo (354-430) Christian church father, philosopher, saint [b. Aurelius Augustinus]

(Misattributed)

Also attributed to St. Benedict, and Brigham Young, Francis Cardinal Spellman (though it predates him). It most likely comes from St. Ignatius Loyola, though that is also subject to some debate.

Variant: "Work as if everything depends on you, and pray as if everything depends on God."