Valmiki Ramayana Kishkindha Kanda Sarga 28 Shloka 64
Original Shlokaउपकारेण वीरस्तु प्रतीकारेण युज्यते। अकृतज्ञोऽप्रतिकृतो हन्ति सत्त्ववतां मनः4.28.64।।
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उपकारेण by a help, वीरः a hero, प्रतिकारेण by helping in turn, युज्यते a suitable step is taken, अकृतज्ञः ungrateful, अप्रतिकृतः one who does not help, सत्त्ववताम् of the virtuous people, मनः heart, हन्ति hurts.
A hero should in return help whoever has helped him. An ungrateful person hurts the heart of the virtuous by not returning the assistance.'