Valmiki Ramayana Bala Kanda Sarga 45 Shloka 19

अथ thereafter, वर्षसहस्रेण in a thousand years, योक्त्रसर्पशिरांसि the hoods of the serpent made as rope, तत्र there, अतिविषम् virulent poison, वमन्ति vomitting, शिला: rocks, दशनै: with teeth, ददंशु: had bitten.

After a thousand years, the hoods of the snake made as rope vomitted venom and started biting the rocks of the Mandara mountain with their teeth.