Srimad Bhagavatham : 5.4.6.
O Maharaja Parikshit, to glorify Maharaja Nabhi, the old sages composed two verses. One of them is this: “Who can attain the perfection of Maharaja Nabhi? Who can attain his activities? Because of his devotional service, the Supreme Personality of Godhead agreed to become his son.”
The words suddhena karmana are significant in this slokam. If work is not carried out in devotional service, it is contaminated by the modes of material nature. That is explained in Bhagavad-gita: yajnarthat karmano nyatra loko yam karma-bandhanah [Bg. 3.9]). Activities performed only for the satisfaction of the Supreme Lord are pure and are not contaminated by the modes of material nature. All other activities are contaminated by the modes of ignorance and passion, as well as goodness. All material activities meant for satisfying the senses are contaminated, and Maharaja Nabhi did not perform anything contaminated. He simply executed his transcendental activities even when performing yajna. Consequently he obtained the Supreme Lord as his son.