Whoever desires to be a Christian must learn to believe this, and to exercise this faith in all his affairs, in physical and in spiritual things, in doing and in suffering, in living and dying, and to cast aside all anxious thoughts and care and throw them cheerfully off. Yet he must not throw them into a corner, as some have vailnly tried to do, for they will not let themselves be strippped of their power so long as they are allowed to dwell in the heart, but you must cast both your heart and your care upon God's back, for He has a strong neck and shoulders, and can well carry them. And, moreover, He bids us cast them upon Him, and the more we cast on Him, the more He is pleased, for He promises that He will bear your burden for you, and everything that concerns you.
'Casting all your care upon Him, for He cares for you.'
O, if a man could learn this casting off of his care, he would know by experience that it is true. But he who does not learn such casting off of his care, must remain a downcast, dejected, defeated, rejected, and hopelessly confused man.
Martin Luther, Sermon on the Third Sunday after Trinity 1544 (WA 22:34f) as cited in Day By Day We Magnify Thee