Valmiki Ramayana Ayodhya Kanda Sarga 16 Shloka 41
Original Shlokaतया सुचरितं देव्या पुरा नूनं महत्तपः।।2.16.41।। रोहिणीव शशाङ्केन रामसंयोगमाप या।
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तया that Sita, शशाङ्केन with the Moon, रोहिणी इव like Rohini, रामसंयोगम् union with Rama, आप got, या such, देव्या by (Sita) devi, पुरा in the past, नूनम् assuredly, महत् great, तपः austerities, सुचरितम् peformed.
Sita was united with Rama like Rohini with the Moon surely because of her great austerities performed in the past. (They observed).