Skandapurana 5.224.8

5.224.8

sarvatraiva pitaḥ proktaṃ pitā tarpaṇakarmaṇi ..
pitrye saṃkalpakāle ca piturakṣayyadāpane .. 8 ..

Everywhere [the vocative] 'O Father' (pitaḥ) is prescribed; [the nominative] 'father' (pitā) in the rite of tarpaṇa; / and in ancestral rites at the time of resolution (saṅkalpa); and [the genitive] 'of the father' (pituḥ) at the offering of the inexhaustible (akṣayya). / 8