Valmiki Ramayana Bala Kanda Sarga 45 Shloka 18
Original Shlokaततो निश्चित्य मथनं योक्त्रं कृत्वा च वासुकिम्। मन्थानं मन्दरं कृत्वा ममन्थुरमितौजस:।।1.45.18।।
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तत: there upon, अमितौजस: those men who were very powerful, मथनम् to churn, निश्चित्य having decided, वासुकिम् Vasuki, योक्त्रम् as rope of the churning rod, कृत्वा having made, मन्दरम् Mandara mountain, मन्थानम् as churning rod, कृत्वा having made, ममन्थु: churned.
Thereupon, having decided to churn (the ocean mind) those very powerful ones, made Vasuki, (the great snake), as a rope and Mandara mountain as the churning rod.And started churning it.