This verse (chaupai) is from the Sundarkanda chapter of Shri Ramcharitmanas.

प्रबिसि नगर कीजे सब काजा। हृदयँ राखि कोसलपुर राजा॥ गरल सुधा रिपु करहिं मिताई। गोपद सिंधु अनल सितलाई॥
This verse (chaupai) comes from Sundarkanda, describing Hanumanji's moment of entering Lanka.
Meaning : Keep Lord Raghunathji, the King of Ayodhya, in your heart, and then enter the city to carry out all your tasks. For him, poison turns into nectar, enemies become friends, the ocean shrinks to the size of a cow's hoof print, and fire turns cool.