Anushasana Parva 25.32

Bathing in the Shashthi lake one acquires merit much greater than what appertains to the gift of food. Ten thousand tirthas and thirty millions of other tirthas come to Prayaga, O chief of Bharata's race, in the month of Magha. He who bathes in Prayaga, with a controlled mind and observing rigid vows all the while the month of Magha, becomes purged of all his sins, O chief of Bharata's race, and attains to heaven.