Matsyapurana 167.66

167.66

vaktramāhṛtavānāśu mārkaṇḍeyaṃ mahāmunim
tato bhagavataḥ kukṣiṃ praviṣṭo munisattamaḥ //

When Lord Viṣṇu thus spoke to that sage, He suddenly swallowed the sage Then the sage rested there in His belly in peace and was desirous of hearing about the truth of eternal Viṣṇu He heard the sound "Haṃsa" there, thus —I am known as the eternal Haṃsa It is I that remains in this great ocean bereft of the Sun and Moon and roam about slowly and again create the world by assuming various bodies—66