Bg 2.19

ya enaṁ vetti hantāraṁ
yaś cainaṁ manyate hatam
ubhau tau na vijānīto
nāyaṁ hanti na hanyate
Synonyms: 
yaḥ — anyone who; enam — this; vetti — knows; hantāram — the killer; yaḥ — anyone who; ca — also; enam — this; manyate — thinks; hatam — killed; ubhau — both; tau — they; na — never; vijānītaḥ — are in knowledge; na — never; ayam — this; hanti — kills; na — nor; hanyate — is killed.
Translation: 
Neither he who thinks the living entity the slayer nor he who thinks it slain is in knowledge, for the self slays not nor is slain.
Purport: 

When an embodied living entity is hurt by fatal weapons, it is to be known that the living entity within the body is not killed. The spirit soul is so small that it is impossible to kill him by any material weapon, as will be evident from subsequent verses. Nor is the living entity killable, because of his spiritual constitution. What is killed, or is supposed to be killed, is the body only. This, however, does not at all encourage killing of the body. The Vedic injunction is mā hiṁsyāt sarvā bhūtāni: never commit violence to anyone. Nor does understanding that the living entity is not killed encourage animal slaughter. Killing the body of anyone without authority is abominable and is punishable by the law of the state as well as by the law of the Lord. Arjuna, however, is being engaged in killing for the principle of religion, and not whimsically.