In Balakanda of Rama-Charitmanasa there's this famous story of Manu and Shatarupa's penance to Vishnu (Rama). Manu asks Vishnu to be born as his son. Shatarupa asks for unending devotion to Vishnu.
Doha - 148 The compassionate Lord then said, "knowing that I am highly pleased with you and recognizing Me as a great donor, ask whatever boon you will.
Doha - 149 "O crest -jewel of donors , O gracious lord , I tell You my sincere wish : I would have a son like You. I can have nothing to conceal from You ."
Chaupala - On seeing his love and hearing his invaluable words, the compassionate Lord said, "Amen But where shall I go find My equal? Myself, O king , shall be a son to you."
Then seeing Satarupa with her hands still folded , He said , O good lady . ask whatever boon you please. "O gracious Lord the boon which the clever king has just asked has appealed to me much.
Doha - 150 "Grant me in Your mercy , O Lord, that very bliss, the same destiny, the same devotion the same attachment to Your feet, the same insight and the same mode of living."
Chaupala - Hearing the soft, pregnant, charming and excellent speech of Satarupa, the gracious Lord gently replied, "Whatever desire you cherish in your mind I have granted; you should have no doubt about it Mother , by My grace your uncommon wisdom shall never fail.
My question is which scripture is this story of Vishnu's boon to Shatarupa?
I have read of Manu's penance in UttaraKhanda (pg 39) of Padma Purana and Vishnu's subsequent avatar as Rama. But which scripture describes Shatarupa's side of the story?