Quotations about:
    zen

You must have three essentials for the investigation of Chan [Zen]. The first is that you must have the foundation of great faith. The second is that you must have a zealous determination. The third is that you must have the feeling of great doubt. If you omit one of these it is like breaking off the leg of a tripod, which ends up becoming a useless vessel.

高峰云、叅禪須具三要 一有大信根
二有大憤志 三有大疑情 苟闕其一
如折足之鼎 終成廢器。

Hyujeong (1520-1604) Korean Seon (Sŏn, Zen) Master [Sosan Taesa, Seosan Daesa, Dae Seonsa]

Mirror of Zen [Samga Gwigam; Samga Kwigom; Seonga Gwigam], ch. 14 [tr. Jorgensen (2012)]
    (Source)

Alternate translations:

For the study of Seon, there are three requirements: (1) having the great root of faith; (2) having great determination, and (3) having great doubt. If you lack one of these, it is like a broken like on a tripod sacrificial vessel. In the end you will discard it.
[tr. Miller (2017)]

There are three essentials to Sŏn meditation. First of all, you must be rooted in Great Faith and Great Confidence. Secondly, one must have Great Anger -- a strong, inwardly-directed, ardent determination to practice. Thirdly, one must have Great Doubt. If one of these is missing, it is like a tripod vessel with one leg cut off -- in the end, it will be of no use.
[Source]

It is well known that Ganhwaseon practitioners must have three things of essential importance: The first is a Foundation of Great Faith (大信根) for the practice which is possible; the second is Great Zealous Determination (大憤志) of practice to attain enlightenment; the third is a Great Feeling of Doubt (大疑情) on the Hwadu. If one of these is lacking, then it is like a tripod pot with a broken foot and is useless.
[Source]