30 Wherefore, that I may briefly embrace the whole matter, let these persons, who from perverse understanding of the Gospel labor to pervert precepts, either take no thought for the morrow, even as the birds of the air; or let them obey the Apostle, as dear children: yea rather, let them do both, because both accord. For things contrary to his Lord, Paul the servant of Jesus Christ would never advise. This then we say openly to these persons; If the birds of the air ye in such wise understand in the Gospel, that you will not by working with your hands procure food and clothing; then neither must ye put any thing by for the morrow, like as the birds of the air do put nothing by. But if to put somewhat by for the morrow, is possibly not against the Gospel where it is said, “Behold the birds of the air, for they neither sow nor reap nor gather into stores;” then is it possibly not against the Gospel nor against similitude of the birds of the air, to maintain this life of the flesh by labor of corporal working.
Source: On the Work of Monks (New Advent)