The chest of the Original Personality of the gigantic form is the luminary planetary system, His neck is the Mahar planets, His mouth is the Janas planets, and His forehead is the Tapas planetary system. The topmost planetary system, known as Satyaloka, is the head of He who has one thousand heads.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
उरःस्थलं ज्योतिरनीकमस्य ग्रीवा महर्वदनं वै जनोऽस्य । तपो रराटीं विदुरादिपुंसः सत्यं तु शीर्षाणि सहस्रशीर्ष्णः
uraḥsthalaṃ jyotiranīkamasya grīvā maharvadanaṃ vai jano'sya | tapo rarāṭīṃ vidurādipuṃsaḥ satyaṃ tu śīrṣāṇi sahasraśīrṣṇaḥ
SB 2.1.28