The RaamaCharitaMaanas states:
Maha-mantra joi japata Maheshu, Kaashi mukta hetu upadeshu
and;
Santata japata Shambhu Avinaashi, Shiv Bhagvaan gyaan guna raashi
Its from Baalkaand. Tulsidaas Gosvami says that Shiv is reciting the maha taaraka mantra 'raam'.