Revākhaṇḍa ascribed to the Vāyupurāṇa (RKV) 88.2

88.2

aṣṭamyāṃ ca site pakṣe caturdaśyāṃ nareśvara /
snāpayetparayā bhaktyā kapilākṣīrasarpiṣā //

On the eighth and on the fourteenth day in the bright fortnight, O lord of men, / one should bathe [him] with supreme devotion using milk and ghee from a brown cow;