Ramayana
पराङ्मुखवधं पापं यः करोत्यसुरेतरः । स हन्ता न गतः स्वर्गं लभते पुण्यकर्मणाम् ॥ 7-8-4 ॥
"He who, though no asura but one other than an asura, commits the sin of slaying one whose face is turned away in flight — such a slayer does not go to heaven, nor does he obtain the worlds of those who perform meritorious deeds."
english translation