A little learning is a dangerous thing;
Drink deep, or taste not the Pierian Spring:
There shallow draughts intoxicate the brain,
And drinking largely sobers us again.

Alexander Pope (1688-1744) English poet

“An Essay on Criticism,” Part 2, ll. 15-18 (1711)
    (Source)

In Greek mythology, the Pierian Spring was sacred to the Muses, representing the metaphorical source of knowledge.

The first line is more commonly paraphrased as "A little knowledge is a dangerous thing."