Skandapurana 1.4.11.145

1.4.11.145

jagatpradhānamiti ca nāma japtvā virājate..
kṛṣṇamūle kṛṣṇaliṃgaṃ nāma cārjitameva ca.. 13.145 ..

shines forth, reciting the name “Source of the Universe”.
At the root of the Kṛṣṇa tree, [Viṣṇu worshipped] a black liṅga, and earned the name [“the Pervader”];