Source

File: wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php

public function check_user_password( $value, $request, $param ) {
		$password = (string) $value;

		if ( empty( $password ) ) {
			return new WP_Error(
				'rest_user_invalid_password',
				__( 'Passwords cannot be empty.' ),
				array( 'status' => 400 )
			);
		}

		if ( false !== strpos( $password, '\\' ) ) {
			return new WP_Error(
				'rest_user_invalid_password',
				__( 'Passwords cannot contain the "\\" character.' ),
				array( 'status' => 400 )
			);
		}

		return $password;
	}