While reading a book "Ancient Science of Mantras" by Om Swami, I have come across a chapter called exceptions. According to this book Shiva at one occasion found a devotee wrongly chanting the Maha Mrityunjay mantra. On the advice of Devi Parvati, he made an exception that some Mantras that include Mantras of Krishna, Mantras from Vedas and His Panchakshari Mantra will be free from any adherence to strict grammatical or initiation rules. The book says that a devotee will only benefit and not be harmed by the chanting of these even if done wrongly. I want to know the document or shastra that says this?
Appreciate your kind help