MN 46: The Greater Discourse on Ways of Undertaking Things
Here, bhikkhus, someone in pain and grief abstains from killing living beings, and he experiences pain and grief that have abstention from killing living beings as condition.
In this sutta, the Buddha admits that following the ethical path isn’t always pleasant. Still, he assures us it’s worthwhile in the end. But the best path of practice is that which is pleasant now and in the future.