Athanasius the Great
Catena Aurea · Lukasevangelium 6
For if ye love them which love you, what thank have ye? for sinners also love those that love them. And if ye do good to them which do good to you, what thank have ye? for sinners also do even the same. And if ye lend to them of whom ye hope to receive, what thank have ye? for sinners also lend to sinners, to receive as much again. But love ye your enemies, and do good, and lend, hoping for nothing again; and your reward shall be great, and ye shall be the children of the Highest: for he is kind unto the unthankful and to the evil. Be ye therefore merciful, as your Father also is merciful.
(Orat. 3. cont. Arian.) That is to say, that we beholding His mercies, what good things we do should do them not with regard to men, but to Him, that we may obtain our rewards from God, not from men.