Skandopanishad

sivasya hridayam vishnuh,vishnOscha hridayam shivah "

"yathA shivamayO vishnurEvam vishnumaya shivah" .

- Skandopanishad

"Shiva dwells in the heart of Vishnu while Vishnu in the heart of shiva.Shiva is a form of Vishnu and Vishnu a form of Shiva. "