From a letter to Margaret, Princess of Anhalt, in 1538:

“We who believe on Him should by all means be confident, for we know that we do not belong to ourselves but to Him who died for us.  Therefore if we are sick, we are not sick unto ourselves; if we are well, we are not well unto ourselves; if we are in troubles, we are not in troubles unto ourselves; if we are glad, we are not glad unto ourselves.  In a word, whatever happens to us does not happen to us but to Him who died for us and has made us His own.  In like manner, when a pious child is sick or suffers from some trouble, it is sicker to the parents than to itself; its trouble strikes the parents harder than the child, because the child is not its own but belongs to the parents….He on whom we believe is almighty.”  (W-Br 8, 190—E 56 xli—SL 21 b, 2221)

cited in Plass, E. What Luther Says (St. Louis, MO: Concordia Publishing House, 2006): 1228.