An "interesting" mantra from "The Sunrise Lands" by S. M. Stirling. The "source" is a character who is former SAS:
I must not lose my temper.
Temper-temper-temper is the bum-killer.
Temper is the little mistake which leads to you lying
On the ground wondering Oi! What´s with all this
spreading pool of blood, then?
I will permit it to pass over me and through me.
And when it has gone past the other bugger
Will be the one bleeding.
Only I will remain, wiping off me knife.
I must not lose my temper.
Temper-temper-temper is the bum-killer.
Temper is the little mistake which leads to you lying
On the ground wondering Oi! What´s with all this
spreading pool of blood, then?
I will permit it to pass over me and through me.
And when it has gone past the other bugger
Will be the one bleeding.
Only I will remain, wiping off me knife.