Shiva Purana
सर्वदेवात्मकश्चासौ सर्वमंत्रमयो गुरुः ॥ तस्मात्सर्वप्रयत्नेन यस्याज्ञां शिरसा वहेत् ॥ २२ ॥
The preceptor is in the form of gods and the mantras. Hence every endeavour shall be made to accept his behest with bent head.
english translation
sarvadevAtmakazcAsau sarvamaMtramayo guruH ॥ tasmAtsarvaprayatnena yasyAjJAM zirasA vahet ॥ 22 ॥
hk transliteration by Sanscript