Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 25
Sanskrit:
सहस्रशिरसं साक्षाद्यमनन्तं प्रचक्षते ।सङ्कर्षणाख्यं पुरुषं भूतेन्द्रियमनोमयम् ॥ २५ ॥
ITRANS:
sahasra-śirasaṁ sākṣādyam anantaṁ pracakṣatesaṅkarṣaṇākhyaṁ puruṣaṁbhūtendriya-manomayam
Translation:
The threefold ahaṅkāra, the source of the gross elements, the senses and the mind, is identical with them because it is their cause. It is known by the name of Saṅkarṣaṇa, who is directly Lord Ananta with a thousand heads.