Pramana: Śāstra Pramāṇa: शास्त्र प्रमाण
Sastra Pramana Source: Mandala-brahmanopanishad (3, 5)
Original Verse: "लक्ष्येऽन्तर्बाह्यायां दृष्ट्औ निमेषोन्मेषवर्जितायां च इयं शांभवी मुद्रा भवति । सर्वतन्त्रेषु गोप्या महाविद्या भवति । तज्ज्ञानेन संसारनिवृत्तिः । तत्पूजनं मोक्षफलदम् ॥ ५ ॥"
Transliteration: "lakṣye'ntarbāhyāyāṃ dṛṣṭau nimeṣonmeṣavarjitāyāṃ ca iyaṃ śāṃbhavī mudrā bhavati | sarvatantreṣu gopyā mahāvidyā bhavati | tajjñānena saṃsāranivṛttiḥ | tatpūjanaṃ mokṣaphaladam ॥ 5 ॥"
Translation: With internal introspection and with the eyes opening out, but devoid of the acts of opening and closing of the eyelids, this Shambhavi-mudra is assumed. This Maha-vidya (Great lore) is, of all the Tantras, what is worthy of being kept a profound secret. With the knowledge (proficiency) of this is attained freedom from the state of worldly existence. (By the Yogin becoming a votary of this), its worship bestows the fruit of liberation.
Apta Pramana Source: "Haṭhapradīpikā of Svātmārāma VII.36"
Original Verse: "अन्तर्लक्ष्यं बहिर्-दृष्टिर्-निमेषोन्मेष-वर्जिता ॥ एषा सा शाम्भवी-मुद्रा सर्वतन्त्रेषु गोपिता ॥३६॥"
Transliteration: "antarlakṣyaṃ bahir-dṛṣṭir-nimeṣonmeṣa-varjitā ॥ eṣā sā śāmbhavī-mudrā sarvatantreṣu gopitā ॥36॥"
Translation: "Fixing the mind on an internal object while keeping the eyes open without blinking, is known as śāmbhavī mudrā, which is a secret preserved in all the tantras. 36."
Additional Source for Sambhavi-Mudra
Pramana: Āpta Pramāṇa - आप्त प्रमाण
Sastra Pramana Source: Haṭhatatvakaumudī (XLIX, 27)
Original Verse: "अन्तर्लक्षविलीनचित्तपवनो योगी यदा वर्त्तते दृष्ट्या निश्चलतारया बहिरधः पश्यन्नपश्यन्नपि मुद्रेयं खलु शाम्भवी भव्ति सा युष्मत्प्रसादात् गुरोः शून्याशून्यविवर्जितं स्फुरति यत्तत्वं पदं शाम्भवम्"
Transliteration: "antarlakṣavilīnacittapavano yogī yadā varttate dṛṣṭyā niścalatārayā bahiradhaḥ paśyannapaśyannapi mudreyaṃ khalu śāmbhavī bhavti sā yuṣmatprasādāt guroḥ śūnyāśūnyavivarjitaṃ sphurati yattatvaṃ padaṃ śāmbhavam |27|"
Translation: When the yogī is in a state where both citta and prāṇa are merged in the internal object of concentration and the gaze is directed outwards but remains steady, seeing yet not seeing, that indeed is śāṃbhavi mudrā, which is received only by the fortunate ones by the grace of the guru, who bestows upon the state of śāṃbhavī, transcending both śūnya and aśūnya.