Skanda Purana 1.4.3.68

1.4.3.68

akāraḥ kathito brahmā ukāro viṣṇurucyate..
makāraśca smṛto rudrastrayaścaite guṇāḥ smṛtāḥ.. 5.68 ..

The syllable ‘a’ is declared to be Brahmā; the syllable ‘u’ is called Viṣṇu; / and the syllable ‘ma’ is remembered as Rudra. These three are also remembered as the guṇas.