You can find the original sermon here, printed in 1686. He is responding to the objection that the term “reward” in Heb. 11:26 implies merit.
On this point, before we finish, we must answer two scruples that can come from these words. The first is whether it is permitted to do good works looking for a reward. The second is whether we can gather as a consequence merit from reward so as to conclude that since our good works have a reward they must be meritorious as  the false Church alleges. But neither the first nor the second have much difficulty in them.
For the first, since the Apostle says so categorically that Moses looked to the reward and our Lord promises on so many places salvation and glory to those who will be faithful in order to cause them to persevere, who can doubt that we can look for it and use looking to the reward as a powerful motive to make us continue in our duty? It is true that this does not always have to be the first or the principal consideration that we make, for then it is certain that our piety would have something mercenary in it. The first reason that should engage us in the practice of piety and virtue surely has to be that of our duty and the obligation that we have to love God and serve Him, that of the excellence of holiness and ugliness of vice. This is true to such an extent that if we had no paradise to hope for or hell to fear that we should not stop fleeing vice and desiring sanctification. But after that motivation, our Lord, accommodating Himself to our weakness,  is willing that we would sustain ourselves in the hope of the glory that He promises us, above all when we look at it as a purely free reward and not as a payment from a master that we can merit but as the liberality of a father who wants to lead His children to the obedience that they owe Him through the testimonies of His favor.
And that’s also how we respond to the second difficulty concerning merit. This is how we see the falseness of the consequence that they want to pull from reward to merit, since Saint Paul teaches us that there is one reward of righteousness and one of grace. Someone would have a right to demand the first on the basis of what they have done. Someone can only claim the other on the basis of the liberality of the giver. I admit that if glory bore the name of a reward in the first sense, we could with more truth deduce merit, although to speak properly, the creature cannot have any merit in relation to the Creator, since he owes to Him absolutely everything that he is and does. But Saint Paul is only speaking of it in the second sense, inasmuch as it’s a reward of grace, which makes him say so often that this glory is a pure gift of the grace of God and not a fruit of our works.
I know that God has put a necessary link between holiness and happiness, righteousness and life, so that the one has to follow after the other infallibly and that without holiness it is impossible to see God. It cannot happen that holiness would not be followed with happiness. But it is also a constant that this consequence only refers to the union of two things, not merit. That’s why Saint Paul declares so expressly “that he considered all the sufferings of the present time could not be compared to the glory that would be revealed in us” (Rom. 8:18). And the nature of the thing shows it clearly enough. Our works are not “ours” but from God without whom we could do nothing. They are never done perfectly, exempted from every fault. They have no proportion with the blessedness that we attain, which is infinite and eternal. On the contrary, our actions and sufferings are short and light. Finally, although we do them, we owe them to God for so many reasons and thus cannot from this standpoint be anything but useless servants. Thus, we could not without absurdity posit any merit of man toward God. Thus when the Apostle speaks of reward, he understands such that is purely an effect of the mercy of God, which he calls elsewhere “a gift of God” and “reward of inheritance” (Rom. 6:23, Col. 3:24) that God puts before the eyes of His children to excite them to do good and encourage them to bear more easily the miseries of this life since they will have such a happy end and be followed with a glorious reward.