CC Madhya 11.104

athāpi te deva padāmbuja-dvaya-
prasāda-leśānugṛhīta eva hi
jānāti tattvaṁ bhagavan-mahimno
na cānya eko ’pi ciraṁ vicinvan
Word for word: 
atha — therefore; api — indeed; te — Your; deva — my Lord; pada-ambuja-dvaya — of the two lotus feet; prasāda — of the mercy; leśa — by only a trace; anugṛhītaḥ — favored; eva — certainly; hi — indeed; jānāti — one knows; tattvam — the truth; bhagavat — of the Supreme Personality of Godhead; mahimnaḥ — of the greatness; na — never; ca — and; anyaḥ — another; ekaḥ — one; api — although; ciram — for a long period; vicinvan — speculating.
“[Lord Brahmā said:] ‘My Lord, if one is favored by even a slight trace of the mercy of Your lotus feet, he can understand the greatness of Your personality. But those who speculate to understand the Supreme Personality of Godhead are unable to know You, even though they continue to study the Vedas for many years.’ ”

This verse is a quotation from Śrīmad-Bhāgavatam (10.14.29). It is explained in the Madhya-līlā, in the sixth chapter, text 84.