CC Madhya 8.264
brahmāke veda yena paḍāila nārāyaṇa
“You have manifested all these transcendental truths in my heart. This is exactly the way Nārāyaṇa educated Lord Brahmā.”
The heart of Brahmā was enlightened by the Supreme Personality of Godhead. This is Vedic information given in the Śvetāśvatara Upaniṣad (6.18):
yo vai vedāṁś ca prahiṇoti tasmai
taṁ ha devam ātma-buddhi-prakāśaṁ
mumukṣur vai śaraṇam ahaṁ prapadye
“Because I desire liberation, let me surrender unto the Supreme Personality of Godhead, who first enlightened Lord Brahmā in Vedic knowledge through Lord Brahmā’s heart. The Lord is the original source of all enlightenment and spiritual advancement.” In this connection one may also refer to Śrīmad-Bhāgavatam 2.9.30-35, 11.14.3, 12.4.40 and 12.13.19.