Bhai Mallo and Bhai Manga were devoted disciples of Guru Nanak Dev ji, who had learned to recite the hymns of Guru Sahib and interpret Gurbani. They came to Guru Nanak Dev ji and sought wisdom and enlightment, which could liberate them from ignorance .
Baba Nanak said If you practice Tasmic Tapsya: dark minded austerities of self mortification, such as sitting near the fire in the hot season or spreading days and nights in water and enduring heat and cold with naked body or leading homeless life, going from one place to another, you have to learn and practice very difficult feats of religious austerities.
On the other hand to control the mind and concentrate on God's presence by listening to the sermons of divine wisdom and singing the sacred hymns of the perfect and interpreting them is Rajasic Tapasya
Bhai Mallo asked "we are told that the more you labour the more are the fruits of your labour, one who works hard gets more wages, and one who does light work gets less, but you say such easy spritual efforts as Kirtan(singing of hymns) and Katha(interpretation of Hymns), yield greater spritual benefits than the difficult austerities practiced by the ascetic?)
Guru Nanak replied "The wood cutters and potters, who carry heavy goods, work very hard, but earn very little compared to the grocery shopkeeper who does far less physical labour, but earns much more money. Cloth merchants and Goldsmiths work even less than grocers, but trade in silks and thousands of rupees and make a lot of profits. The jewellers lead an easy and peaceful life, but by trading in pearls and gems they earn more than all others. He who serves in the shops of jewellers, becomes after some experience, a connoisseur of gems. So all ascetic efforts and fasting and self mortification put a great strain on the body and mind of those who practice them, but very little true religious knowledge, experience and illumination is attained by them. By reading, reciting, singing and understanding interpreting the sacred hymns of the Gurus and the mystic saints for the benefits of others, the discipline of true worship and meditation is acquired and true divine knowledge and illumination is achived. This is "Sattvic Tapasya". Thus, the Sattvic Tapasya in the Sikh tradition yields the maximum benefits". After listening to this sermon Bhai Mallo studied and interpreted the hymns of the Pothi Sahib and Bhai Manga listened to his interpretations with single minded devotions. Both of them performed kirtan. Whosever listened to them was greatly inspired by spritual fervour and gave up the life of sin and evil and depended solely on his honest earnings.