Midrash Ruth Rabbah 5:6

Rabbi Isaac ben Marion said: This verse can teach us that if one is about to perform a good deed, they should do it with all their heart. For had Reuben known that Scripture would record of him, “And Reuben heard it, and delivered him out of their hand (Genesis 37:21), he would have borne Joseph on his shoulder to his father; and had Aaron known that Scripture would record of him, “And also, behold, he comesforth to meet you” (Exodus 4:14), he would have gone forth to meet him [Moses] with timbrels and dances. And had Boaz known that Scripture would record of him, “And he handed her parched corn, and she ate and was satisfied and had some leftover,” he would have fed her with fatted calves. [Translation by Soncino. Edited for gender neutrality and modernity by AJWS]