Gayatri Mantra
Brahmarshi Vishvamitra (Sanskrit विश्वामित्र viśvā-mitra "all-friend") is one of the most venerated rishi's or sages of ancient times in India. He is also credited as the author of most of Mandala 3 of the Rigveda, including the revered great Gayatri Mantra. It is a mantra cum prayer and is found in all the three Vedas; Rig, Yajur and Sama Veda. Veda's clearly state that anyone can chant this Mantra, and gain its benefits. The Puranas mention that only 24 Rishis since antiquity have understood the whole meaning of, and thus wielded the whole power of, the Gayatri Mantra. Sage Vishvamitra is supposed to be the first, and Sage Yajnavalkya the last.
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