Bhagavatapurana 10.70.5

5. Atman who is but one (individual whole), self-luminous Supreme Light unconditioned, eternal and by virtue of His own nature eternally destitute of the touch of Avidya, known as the Brahman and whose existence and blissful nature is indicated by His own powers that bring about the origin, destruction, etc., of this Universe.

6. Then as laid down in sastras He the foremost among the righteous bathes in pure water, puts on a pair of (washed) clothes practises contemplation in the course of Sandhya and other observances, pours oblations into fire and silently repeats and contemplates Brahman in the from of Gayatri.