1 Corinthians 16:9
Will you have mention also of the resources out of which you should make your deposits, so as in this respect also to make this kind of contribution easy? The handicraft man, for instance, the sandal-maker, or the leather-cutter, or the brass-founder, or any other artificer—when he sells any article of his trade, let him give the first-fruits of its price unto God: let him cast in a small portion here, and assign something to God out of his portion, though it be rather scanty. For neither do I ask any great thing; but so much as the childish ones among the Jews, full as they are of innumerable evils, just so much let us cast in, we who look forward to heaven. And this I say not as laying down a law, neither as forbidding more, but as recommending a deposit of not less than a tenth part. And this also do thou practise not in selling only, but also in buying and receiving a recompense. Let those also who possess land observe this law in regard to their rents: yea, let it be a law for all who gather their incomes in an honest way. For with those who demand usury I have no concern, neither with soldiers who do violence to others and turn to their own advantage their neighbors' calamities. Since from that quarter God will accept nothing. But these things I say to those who gather their substance by righteous labor.
Yea, and if we establish ourselves in this kind of habit, we are ever after stung by our conscience if ever we omit this rule; and after a while we shall not even think it a hard thing; and by degrees we shall arrive at the greater things, and by practising how to despise wealth, and by pulling up the root of evils, we shall both pass the present life in peace, and obtain the life to come; which may it be the portion of us all to attain unto, etc. etc.
Source: Homilies on First Corinthians (New Advent)