Monks, there are two persons that cannot easily be repaid.

What two?

One’s mother and father.

Even if one should carry about one's mother on one shoulder and one’s father on the other, and (while doing so) should have a life span of a hundred years, live for a hundred years; and if one should attend to them by anointing them with balms, by massaging, bathing, and rubbing their limbs, and they even void their urine and excrement there, one still would not have done enough for one's parents, nor would one have repaid them.

Even if one were to establish one's parents as the Supreme Lords and Rulers over this great earth abounding in the seven treasures, one still would not have done enough for one's parents, nor would one have repaid them.

For what reason?

Parents are of great help to their children; they bring them up, feed them, and show them the world.

But, Monks, if, when one's parents lack confidence, one encourages, settles, and establishes them in confidence; if, when one's parents are unwholesome, one encourages, settles, and establishes them in virtuous behavior (moral conduct); if, when one’s parents are selfish, one encourages, settles, and establishes them in generosity; if, when one's parents are unwise, one encourages, settles, and establishes them in wisdom: in such a way, one has done enough for one’s parents, repaid them, and done more than enough for them.

(Reference: AN 2.33)