Devibhagavatapurana 1.5.101

1.5.101

devyuvāca |
gṛhaṃ gaccha mahābhāga kuru rājyaṃ yathāsukham |
hayagrīvādṛte mṛtyurna te nūnaṃ bhaviṣyati ||

O highly fortunate one! “Go home and govern your kingdom at your ease; death won't occur to you from any other beings then from one who is horse-faced.” Thus granting the boon, the Devî vanished. Becoming very glad on getting this boon, Hayagrîva went to his residence. Since then the wicked Daitya is troubling very much all the Devas and Munis. There is none in the three worlds to kill him. So let Visvakarmâ take a horse's head and fix it on the headless body of Visnu. Then Bhagavân Hayagrîva will slay the vicious wicked Asura, for the good of the Devas”.