Shiva Purana
पंचोपचारैस्संपूज्य शिरसा श्रीगुरुम्बुधः ॥ प्रणवं श्रीचतुर्थ्यंतं नमोंतं प्रणमेत्ततः ॥ ४५ ॥
Having worshiped him with the five rituals he bowed his head to the Śrī Guru. Then one should offer obeisances to the fourth oṁkāra of Śrī.
english translation
paMcopacAraissaMpUjya zirasA zrIgurumbudhaH ॥ praNavaM zrIcaturthyaMtaM namoMtaM praNamettataH ॥ 45 ॥
hk transliteration by Sanscript