Hari-Hara abheda - Sanskrit
Sridhara Swamin, in his invocation to the Bhagavatam says:

माधवोमाधवावीशौ सर्वसिद्धिविधायिनौ। वन्दे परस्परात्मानौ परस्परनुतिप्रियौ॥
I bow to Mādhava and Umādhava (Shiva) who are both 'Isha-s' Supreme Lords. They are capable of bestowing all accomplishments (to their devotees). They are both the selves of each other and both love to engage in the stuti of each other.

And Veda Vyasa in the Mahabharata:
रुद्रो नारायणश्चैव सत्त्वमेकं द्विधा कृतम्।
लोके चरति कौन्तेय व्यक्तिस्थं सर्वकर्मसु।। 12-350-27a 12-350-27b.

This idea of Hari-Hara abheda is admitted in the Smarta samparadaya. The smartas of Karnataka and Andhra chant these mantras as part of their daily three-time sandhyavandanam:

शिवाय विष्णुरूपाय शिवरूपाय विष्णवे । शिवस्य हृदयं विष्णुः विष्णोश्च हृदयं शिवः ॥८॥ यथा शिवमयो विष्णुरेवं विष्णुमयः शिवः । यथान्तरं न पश्यामि तथा मे स्वस्तिरायुषि ॥९॥ यथान्तरं न भेदाः स्युः शिवकेशवयोस्तथा ।[Skandopaniṣat]

[(obeisance to Śiva who is of the form of Viḷṣṇu, and to Viṣṇu of the form of Śiva. Śiva's heart (self) is Viṣṇu and Viṣṇu's self is Śiva. Just as Viṣṇu is fully verily Śiva, so too Śiva is fully verily Viṣṇu. As I do not see any difference between them, let me be prosperous and long-lived. Let there be no difference between Śiva and Keśava.

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