Serving the Guru
(1) Shvetashvatara Upanishad (6/23) :
Advocates worship to the Guru in the same manner as the deity - God, to attain all there is to attain on the path of God-realization:
Yasya deve para bhaktir yatha deve tatha gurau |
Tasyaite kathitaa hi arthaaha prakashante mahatmanaha ||
(2) Bhagavad Gita (4/34) :
The disciple should humbly pose questions to the Guru and please him by serving him. He will then impart the knowledge of God, so ordain the wise sages.
(3) Shrimad Bhagvatam :
Bhagwan Rushabhdeva advocates his sons: Obeying the Anuvrutti - unvoiced wishes - of God and Guru is devotion.
(4) Skanda Purana - Guru Gita
Gurubrahma Guruvishnu Gururdevo Maheshwaraha |
Guruhu sakshaat Parambrahman tasmai Shrigurave namaha ||
"The guru is Brahma, Vishnu and Mahesh (Shiva), veneration to the Guru who is Parabrahman manifest."
The second line of the couplet does not literally mean that the Guru becomes Parabrahman - God, rather he is venerated as if God is manifesting through him.
(5) Another famous verse known to many Hindus:
Guru Govind donu khade, kisko laagu paay,
Balihari Gurudevaki jinhe Govind diyo bataay.
Guru and God are both standing before me, whom shall I bow first? Thanks to the Guru who has shown me the God.
Regards,