Skandapurana 1.3.6.113

1.3.6.113

tato'sau tapasā yuktaḥ sārddhaṃ tairdevatāgaṇaiḥ..
sagaṇaṃ rāvaṇaṃ rāmaḥ ṣaḍbhirmāsairajīhanat..
viṣṇunā ghātitaḥ śastraiḥ śivasārūpyamāptavān.. 8.113 ..

Then, endowed with tapas, together with those hosts of deities,
Rāma killed Rāvaṇa along with his host within six months.
Slain by Viṣṇu with weapons, he attained conformity of form with Śiva.