Valmiki Ramayana · Ayodhya Kanda · Sarga 5
इत्युक्त्वा स तदा राममुपवासं यतव्रतम्। मन्त्रवत्कारयामास वैदेह्या सहितं मुनिः
ityuktvā sa tadā rāmamupavāsaṃ yatavratam| mantravatkārayāmāsa vaidehyā sahitaṃ muniḥ
The ascetic (Vasistha) having thus spoken to Rama made him undertake fast along with Sita in accompaniment with sacred hymns.
— M.N. Dutt (1891)
Commentary
Vasistha performs the sacred hymns and makes Rama and Sita undertake the fast together. The couple fasts as one — their first act as king and queen is a shared renunciation. The ritual binds them not only to each other but to the divine order they are about to serve.
मन्त्रदम्पती