Valmiki Ramayana Bala Kanda Sarga 21 Shloka 20
Original Shlokaतेषां निग्रहणे शक्तस्स्वयं च कुशिकात्मज:। तव पुत्रहितार्थाय त्वामुपेत्याभियाचते।।1.21.20।।
1 source checked for this shloka
कुशिकात्मज: son of Kusika, स्वयं च by himself, तेषाम् of those rakhsasas, निग्रहणे in repressing, शक्त: capable, तव your, पुत्रहितार्थाय welfare of your son, त्वाम् you, उपेत्य having reached, अभि याचते begs.
Son of Kusika, even though capable of repressing those rakhsasas by himself, it is for the welfare of your son that he is here requesting you to spare him".