Shiva Purana
आधारशक्तिमारभ्य त्रितत्वांतमयं पदम् ॥ विचिन्त्य मध्यतस्तस्य दहरं व्योम भावयेत् ॥ ६८ ॥
After thinking about the region from the Ādhāra-Śakti to the three Tattvas the Dahara Vyoman (the ether-heart) shall be imagined and conceived in the middle.
english translation
AdhArazaktimArabhya tritatvAMtamayaM padam ॥ vicintya madhyatastasya daharaM vyoma bhAvayet ॥ 68 ॥
hk transliteration by Sanscript