Who said "I don't know how to beat others. I know how to beat myself."?

Yamamoto Tsunetomo (1659–1719). Source: Hagakure, Book 1.

What is the original Japanese?

人に勝つ道はしらず我に勝つ道を知りたり. Translated from the original Japanese by Spartan Surfers.

I don't know how to beat others. I know how to beat myself.

The source

"I don't know how to beat others. I know how to beat myself." Yamamoto Tsunetomo said it in Hagakure, Book 1. Translated from the original Japanese by Spartan Surfers.

"人に勝つ道はしらず我に勝つ道を知りたり"

Original Japanese, Hagakure, Book 1. Japanese: Hagakure, vol. 1 (1937 printing), NDL digital text; Saying of Yagyu quoted in Hagakure.

Who said it

Yamamoto Tsunetomo (1659–1719). Samurai turned monk whose spoken reflections were written down as Hagakure, Hidden by Leaves.

Questions

Who said "I don't know how to beat others. I know how to beat myself."?: Yamamoto Tsunetomo (1659–1719). Source: Hagakure, Book 1.
What is the original Japanese?: 人に勝つ道はしらず我に勝つ道を知りたり. Translated from the original Japanese by Spartan Surfers.