I ended up finding this sutta while reading the jatakas, sir.
- Paniya Jātaka: “Brethren, there is no such thing as a petty sin. A Brother must check all sins as they each arise. Wise men of old, before the Buddha came, subdued their sins and attained the knowledge of a Pacceka-Buddha.” With these words, he told them a story of the past. Once upon a time, when Brahmadatta was king in Benares, there were two friends in a certain village in the kingdom of Kasi. These had gone afield, taking with them vessels for drinking, which they laid out of the way as they broke the clods, and when they were thirsty, went and drank water out of them. One of them, on going for a drink, husbanded the water in his pot and drank out of the pot of the other. In the evening, when he came out from the woodland and had bathed, he stood thinking. “Have I done any sin today,” thought he, “either by the door of the body, or any other?” Then he remembered how he drank the stolen water, and grief came upon him, and he cried, “If this thirst grows upon me, it will bring me to some evil birth! I will subdue my sin.” So with this stolen draught of water for cause, he gradually acquired supernatural insight, and attained the knowledge of a Pacceka-Buddha; and there he stood, reflecting upon the knowledge which he had attained.
- Another man, who also lived in the village of Kasi, a land-owner, was sitting in the bazaar when he saw a man approach leading his wife. Seeing her (and she was a woman of surpassing beauty) he broke the moral principles, and looked upon her; then again he thought, “This desire, if it increases, will cast me into some evil birth.” Being exercised in mind, he developed supernatural insight, and attained the knowledge of a Pacceka Buddha; then poised in the air, he delivered a discourse, and he also went to the Nandamula cave.
Another source:Paniya Jataka (#459)
Then he told them this story as an example of how contemplating small sins can lead to great attainment. When the Buddha was finished, all five hundred disciples became arahants.
I think it’s Anapanasati right?