CC Madhya 25.105

jñānaṁ parama-guhyaṁ me
yad vijñāna-samanvitam
sa-rahasyaṁ tad-aṅgaṁ ca
gṛhāṇa gaditaṁ mayā
jñānam — knowledge; parama — extremely; guhyam — confidential; me — of Me; yat — which; vijñāna — realization; samanvitam — fully endowed with; sa-rahasyam — with mystery; tat — of that; aṅgam — supplementary parts; ca — and; gṛhāṇa — just try to take up; gaditam — explained; mayā — by Me.
“ ‘Please hear attentively what I shall speak to you, for transcendental knowledge about Me is not only scientific but full of mysteries.

This is a quotation from Śrīmad-Bhāgavatam (2.9.31). For an explanation see Ādi-līlā, chapter one, text 51.