Kudos to a monk named Abhipañño for this one:
Monks, even if bandits were to sever you savagely limb by limb with a two-handled saw, he who gave rise to a mind of hate toward them would not be carrying out my teaching. Herein, monks, you should train thus: ‘Our minds will remain unaffected, and we shall utter no bitter words; we shall abide compassionate for their welfare, with a mind of loving-kindness, never in a mood of hate. We shall abide pervading them with a mind imbued with loving-kindness; and starting with them, we shall abide pervading the all encompassing world with a mind imbued with loving-kindness, abundant, exalted, immeasurable, without hostility, and without ill will.’ That is how you should train, monks.
“Monks, if you keep this advice on the simile of the saw constantly in mind, do you see any course of speech, trivial or gross, that you could not endure?” - “No Venerable Sir.” – “Therefore, monks, you should keep this advice on the simile of the saw constantly in mind. That will lead to your welfare and happiness for a long time.”
MN 21: Kakacupama Sutta, I 126-27, 129