The following text is part of one of the Śaiva Upaniṣadas, which are included in the Muktika canon - Rudrahṛdaya Upaniṣad. It is associated with Kṛṣṇa-Yajurveda and composite in the form of a dialogue between the sage Vyāsa and his son and disciple Śuka. Rudrahṛdaya literally means "the heart of Rudra". In this Upaniṣad, Rudra is described as the Paramātman which contains all the devas within Himself. The central and main statement of the Rudrahṛdaya Upaniṣad is contained in its fourth śloka:
सर्वदेवात्मको रुद्रः सर्वे देवाः शिवात्मकाः ।
sarvadevātmako rudraḥ sarve devāḥ śivātmakāḥ |
Rudra is Soul (or Self) of all devas, and all devas are Śiva in their essence.
The text, states Śakya, is the only Upaniṣad that presents the composite merged form of Rudra-Umā as all truth and reality, and emphasizes this hermaphrodite-style union aspect by presenting the unison in other combinations such as Brahmā-Vāṇī and Viṣṇu-Lakṣmī. The date and author of Rudrahṛdaya Upaniṣad are unknown.
Ślokas 16 - 25 are often used as an independent prayer text describing and glorifying the Eternal Union of Rudra and Umā (or Śiva and Śaktī), represented in the form of a Śivaliṅgam. It is said that one who daily reads this text with devotion gets rid of all, even the most serious sins. Sometimes it is also read during the Śivaliṅga puja.
Rudrahṛdaya Upaniṣad (16 - 25)
श्रीरुद्र रुद्र रुद्रेति यस्तं ब्रूयाद्विचक्षणः ॥
śrī rudra rudra rudreti yastaṃ brūyādvicakṣaṇaḥ
"Sri Rudra, Rudra, Rudra" - the wise one who utters this,
कीर्तनात्सर्वदेवस्य सर्वपापैः प्रमुच्यते ।
रुद्रो नर उमा नारी तस्मै तस्यै नमो नमः ॥
kīrtanāt sarvadevasya sarvapāpaiḥ pramucyate
rudro nara umā nārī tasmai tasyai namo namaḥ
By gloryfication all the devas, he is freed from all his sins.
Rudra is man, Umā is woman, - Prostrations to Him and Her!
रुद्रो ब्रह्मा उमा वाणी तस्मै तस्यै नमो नमः ।
रुद्रो विष्णुरुमा लक्ष्मीस्तस्मै तस्यै नमो नमः ॥
rudro brahmā umā vāṇī tasmai tasyai namo namaḥ
rudro viṣṇurumā lakṣmīstasmai tasyai namo namaḥ
Rudra is Brahmā, Umā is Sarasvatī, - Prostrations to Him and Her!
Rudra is Viṣṇu, Umā is Lakṣmī, - Prostrations to Him and Her!