The main source of Mudra are Gherandya Samhita, Hathyoga Pradipika. Gherandya Samhita is written by Sage Gherandya and Hathyoga Pradipika is written by Swami Pt. Swatmaram from Nath Tradition. Thenafter was more work on those topic through Swami Satyanand Saraswati. He was the founder of Bihar School of Yoga. Mudrās are a fundamental form of yoga practice; the most famous book published by the Bihar School of Yoga is called Asana, Pranayama, Mudrā, Bandha.