system/Validation/Validation.php
public function setRule(string $field, string $label = null, string $rules, array $errors = [])
	{
		$this->rules[$field] = [
			'label' => $label,
			'rules' => $rules,
		];
		$this->customErrors  = array_merge($this->customErrors, [
			$field => $errors,
		]);

		return $this;
	}
public function setRule(string $field, string $label = null, string $rules, array $errors = [])
	{
		$this->rules[$field] = [
			'label' => $label,
			'rules' => $rules,
		];
		$this->customErrors  = array_merge($this->customErrors, [
			$field => $errors,
		]);

		return $this;
	}
Sets an individual rule and custom error messages for a single field.

The custom error message should be just the messages that apply to
this field, like so:

[
'rule' => 'message',
'rule' => 'message'
]

@param string      $field
@param string|null $label
@param string      $rules
@param array       $errors

@return $this