Revākhaṇḍa ascribed to the Vāyupurāṇa (RKV) 20.55

20.55

patnīsaṃyojanaṃ cānyaddaivakarma tataḥ param /
mānuṣyaṃ pitṛkarma syāddaśamāṣṭāsu śobhane //

joining with a wife, and then other rites for the gods; / the human and ancestral rites should be eighteen, O fair one;