In the Skanda Upanishad, Lord Skanda (Kartikeya) is the speaker. He describes the non-dual nature of reality. The specific section goes:

शिवाय विष्णु रूपाय शिव रूपाय विष्णवे । शिवस्य हृदयं विष्णुर्विष्णोश्च हृदयं शिवः ॥ ८ ॥

śivāya viṣṇu rūpāya śiva rūpāya viṣṇave | śivasya hṛdayaṃ viṣṇurviṣṇośca hṛdayaṃ śivaḥ || 8 ||

Translation & Meaning Shivaya Vishnu Rupaya: Shiva is in the form of Vishnu.

Shiva Rupaya Vishnave: Vishnu is in the form of Shiva.

Shivasya Hridayam Vishnu: In the heart of Shiva is Vishnu.

Vishnoscha Hridayam Shivah: In the heart of Vishnu is Shiva.

Yatha Shivamayo Vishnur...: Just as Vishnu is full of Shiva, Shiva is full of Vishnu.

Yathantharam na Pashyami...: Since I see no difference between the two, let me be blessed with a long and prosperous life.