Mere Christianity, in The Complete C. S. Lewis Signature Classics

pp. 52-53

“Charity—giving to the poor—is an essential part of Christian morality: in the frightening parable of the sheep and the goats it seems to be the point on which everything turns . . .I do not believe one can settle how much we ought to give.  I am afraid the only safe rule is to give more than we can spare.  In other words, if our expenditures on luxuries, comforts,, amusements, etc., is up to the standard common among those with the same income as our own, we are probably giving away too little.  If our charities do not at all pinch or hamper us, I should say they are too small.  There ought to be things we should like to do and cannot do because our charities expenditure excludes them.”