There are two extremes that are not to be indulged in

by one who has gone forth. Which two?

That which is devoted to sensual pleasure

with reference to sensual objects:

base, vulgar, common, ignoble, unprofitable;

and that which is devoted to self-affliction:

painful, ignoble, unprofitable.

Avoiding both of these extremes,

the Middle Way realized by the Tathagata

—producing vision, producing knowledge—

leads to calm, to direct knowledge, to self-awakening, to liberation.

―Buddha Shakyamuni, Dharmachakrapravartana Sutra

Source: Avoid Both