Ramayana
लोके च मामिका । एकैकस्योपकारस्य प्राणान्दास्यामि ते कपे । नरः प्रत्युपकाराणामापस्त्वायाति पात्रताम् ॥ 7-40-22 ॥
[So long shall my story endure] in the world. For each single act of service I would give up my own life for you, O monkey; yet a man becomes worthy of repaying kindness only when calamity befalls his benefactor.
english translation