Yoga Sutras of Patanjali translated by Yogi Kalinath - Chapter 1, Verse 26
Pada 1, Sutra 26
Sa purvesam api guruh kalenanavacchedat
Ishwara, being unlimited by time, is also the Teacher of the ancients.
Ishwara or God is the First Teacher of humanity and therefore the Guru of all Gurus, the Teacher of all teachers from the beginning to the end of time as He Himself is the embodiment of Knowledge that is unsurpassed. Highest praise therefore to the Light of Knowledge and Divine Wisdom embodied in the Guru Spirit within and without.
To the (non-dualistic) Tantricks, Brahman is the Absolute, Ineffable One, whose Consciousness Expression is Shiva. As Knowledge is the expression of Consciousness itself, Shiva is therefore the Original Teaching Guru of the Yogins.