He must … observe silence and refrain from idle talk. For the tongue is a smoldering fire, and excess of speech a deadly poison. Material fire consumeth the body, whereas the fire of the tongue devoureth both heart and soul. The force of the former lasteth but for a time, whilst the effects of the latter endureth a century.
(Bahá’u’lláh, Gleanings from the Writings of Bahá’u’lláh, p. 264)
Poison is harmful to man. It is the nature of man to find enjoyment in that which is gratifying to his senses; if he pursue this path he subverts his individuality to such a degree that the poison of darkness which was the means of death becomes the means of his existence and his nature becomes so degraded and his individuality so deflected that his one purpose in life will be to obtain the death-dealing drug.
(‘Abdu’l-Bahá, Divine Philosophy, p. 132)
To nurse a grievance or hatred against another soul is spiritually poisonous to the soul which nurses it.
(Universal House of Justice to an individual believer, 5 January 1992)