Matsya Purana 188.92

188.92

amareśvaradevasya parvatasya ubhe taṭe.
tatra tā ṛṣikoṭyastu tapastapyanti suvrata! .. 188.92 ..

Such blessed one after being liberated from all actions and endowed with supreme intellect go and reside in the region of Rudra till the end of the cycle—91 Crores of Ṛiṣis practise asceticism on the either side of Amaresvara. This Mount Amarakantaka is one yojana in extent all round. Those who bathe in the sacred Narmadâ with or without an object in view, are liberated from all their sins and go to the realm of Rudra.—92-94.