Skandapurana 7.20.15

7.20.15

dhṛtakapālamṛya bhairavarūpiṇo rudrasya bhaikṣārthaṃ tato brahālokagamanam, tatromiti vyāharatastasya vadanāt sarasvatyāḥ prādurbhāvaḥ, kapālasaṃsthāpanam, mahiṣarūpiṇo hālāhalāhvayasyāsurasya vināśaḥ, kapālamātṝṇāmutpattiḥ, mahākapālasarasastotrasya cotpattiḥ .

Rudra in the form of Bhairava, carrying the skull, goes from there to the realm of Brahmā for alms; while he utters 'Om' there, the manifestation of Sarasvatī from his mouth; the placing down of the skull; the destruction of the demon named Hālāhala who took the form of a buffalo; the origin of the skull-Mothers; and the origin of the praise of the Great Skull Lake.