Do not strike at the weak points of others or do anything which will cause them suffering. In the same way, do not recite destructive mantras which will harm non-human beings.<
From Enlightened Courage, by Dilgo Khyentse Rinpoche. Copyright 1993 by Editions Padmakara (Padmakara Translation Group). Published and distributed by Snow Lion Publications. Used by permission.
This site provides an on-line database of commentaries on the Tibetan Buddhist meditation practices of lojong (Mind Training) and tonglen.