Devadas is singing the Sri Nrisimha Maha Mantra. This mantra is a Vishnu Kirtan. To sing along you find here the text:
Ugram Viram Maha-vishnum 'Jvalantam Sarvato-mukham /
Nri-sinham Bhishanam Bhadram 'Nrityu-nrityum Namany Aham // 1 //
Namas Te Nara-sinhaya ' Prahlada-hlada-dayine /
Hiranya-kashipor Vaksah- ' Shila-tanka-nakhalaye // 2 //
Ito Nri-sinhah Parato Nri-sinho ' Yato Yato Yami Tato Nri-sinhah /
Bahir Nri-sinho Hridaye Nri-sinho ' Nri-sinham Adim Sharanam Prapadye // 3 //
You can find the text with further explanations to this mantra here.
More information about Yoga, Meditation and Ayurveda can be found on the websites of Yoga Vidya.
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You can get to other mantra's here.