Valmiki Ramayana Yuddha Kanda Sarga 34 Shloka 6

त्वम् you, गगनम् in the sky, अपिच even, रसातलम् underworld, गन्तुम् , समर्था capable, अकर्तव्यम् that which can't be done, मदन्तरे for my sake, ते you, कर्तव्यम् duty, एवगच्छामि also known to me.

"You are capable of flying through the sky or even going to the underworld. For my sake you will do what cannot be done also and that it is your bounden duty, is also known to me."