Skandapurana 6.3.20.55

6.3.20.55

patnīsaṃyojanaṃ cānyaddaivakarma tataḥ param .
mānuṣyaṃ pitṛkarma syāddaśamāṣṭāsu śobhane .. 20.55 ..

and next, union with a wife, and then the rites for the gods; / the human rites, the ancestral rites, in the eighteenth, O fair one;