Valmiki Ramayana Kishkindha Kanda Sarga 34 Shloka 10
Original Shlokaपूर्वं कृतार्थो मित्राणां न तत्प्रतिकरोति यः। कृतघ्नस्सर्वभूतानां स वध्यः प्लवगेश्वर।।4.34.10।।
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प्लगेश्वर O lord of monkeys, पूर्वम् first, कृतार्थः is fulfilled, यः whoever, मित्राणाम् to the friends, तत् that, प्रतिकरोति does not help in turn, कृतघ्नः an ungrateful one, सः he, सर्वभूतानाम् to all beings, वध्यः deserves to be killed.
'O lord of monkeys he who, having accomplished his task first with the help of his friend, does not return the help is ungrateful. Such a person deserves to be killed.It applies to all beings.