Shiva Purana
मूर्त्यात्मसु शिवाः केचिदध्वनां मूर्धसु स्थिताः ॥ मध्ये महेश्वरा रुद्रास्त्वर्वाचीनपदे स्थिताः ॥ ६५ ॥
Among those who have attained identicality, some are Śivas. They are stationed on the top of the Adhvas (pathways). Maheśvaras are stationed in the middle; Rudras in the lower region.
english translation
mUrtyAtmasu zivAH kecidadhvanAM mUrdhasu sthitAH ॥ madhye mahezvarA rudrAstvarvAcInapade sthitAH ॥ 65 ॥
hk transliteration by Sanscript