Mudgalapurana 2.1.22

2.1.22

nilīya tasminnabdānte nābhipadmādviniḥsṛtaḥ |
varadānena viṣṇośca putratvamagamadvidhiḥ ||

Having remained concealed within him, at the end of a year he emerged from the navel-lotus; / and through the granting of a boon by Viṣṇu, the Creator became his son.