A true teacher is compared to God, by his students. The word guru means remover of darkness. Homage is paid to God, the Guru of Guru. Guru mantras originate from various sources.

Guru Stotram from the Vishamsara Tantram (of which only portions survive today) the Guru Gita that is part of the Skanda Purana. The Guru Stotram is also ascribed to Adi Shankaracharya.

Gurur-Brahmaa gurur-vishnu
Gurur-devo Maheshwarahah
Gurur saakshaat para-brahmaa
Tasmai shree-guruve namaha.
Guru is the creator (Brahmaa)
Guru is the Preserver (Vishnu)

Gurudeva is the Destroyer (Maheshwara).
Guru is the absolute (singular) Lord himself,
Salutations to that Sri Guru.

Akhanda mandalaakaram Vyaaptam yena charaa-charam
Tatpadam darshitam yena Tasmai shree-guruve namaha.

Which pervades the entire unbroken form of the circle (of creation), moving and unmoving. To that beautiful and benevolent Guru through whom that state was revealed (to me), salutations. – Translation by Swami Veda Bharati.

My Lord is the lord of the universe; my teacher is the teacher of the entire universe; and myself is the self of all. My salutations at the lotus-feet of such a Guru, who has revealed such knowledge to me.

All the Vedas are mastered, and they are ever present on the lips; likewise, the knowledge of other sciences is also well gained. The ability to write prose as well as verse is present too. Even then of what ultimate benefit is all this, if one’s mind does not devoutly rest upon the lotus feet of the guru, (by whose grace alone can one’s soul be liberated)? Of what use, of what indeed. – Guru Bhakti Stotram. Translation by Swami Brahmananda.

Salutations to the Guru, who removes darkness of ignorance from our blind (inner) eyes by applying the Collyrium (kohl) of the light of knowledge.
By whom our inner eyes are opened; Salutations to that Guru. – Guru Gita: Green Message.org.

Dear Ones, the revelation is that only the lotus feet of the Supreme Guru, God can liberate us.

Aim Hrim Klim