The Spear
Satti Sutta  (SN 20:5)

Staying near Sāvatthī. “Monks, suppose there were a sharp-bladed spear, and a man were to come along saying, ‘With my hand or fist I will bend back this sharp-bladed spear, fold it in two, and roll it up.’ What do you think? Would that man be able with his hand or fist to bend back that sharp-bladed spear, fold it in two, and roll it up?”

“No, lord. Why is that? Because a sharp-bladed spear isn’t easy to bend back, fold in two, or roll up. The man would simply reap his share of trouble & vexation.”

“In the same way, monks, when a monk’s awareness-release through good-will is cultivated, developed, pursued, given a means of transport, given a grounding, steadied, consolidated, & well-undertaken, any non-human being who would think of deranging that monk’s mind would simply reap his share of trouble & vexation.

“Thus you should train yourselves: ‘Our awareness-release through good-will will be cultivated, developed, pursued, given a means of transport, given a grounding, steadied, consolidated, & well-undertaken. That’s how you should train yourselves.”

See also: MN 21; AN 3:66; AN 3:101; AN 11:16; Iti 22; Iti 27