wp_authenticate_username_password

function wp_authenticate_username_password( $user, $username, $password ) {
	if ( $user instanceof WP_User ) {
		return $user;
	}

	if ( empty( $username ) || empty( $password ) ) {
		if ( is_wp_error( $user ) ) {
			return $user;
		}

		$error = new WP_Error();

		if ( empty( $username ) ) {
			$error->add( 'empty_username', __( '<strong>Error</strong>: The username field is empty.' ) );
		}

		if ( empty( $password ) ) {
			$error->add( 'empty_password', __( '<strong>Error</strong>: The password field is empty.' ) );
		}

		return $error;
	}

	$user = get_user_by( 'login', $username );

	if ( ! $user ) {
		return new WP_Error(
			'invalid_username',
			__( 'Unknown username. Check again or try your email address.' )
		);
	}

	/**
	 * Filters whether the given user can be authenticated with the provided $password.
	 *
	 * @since 2.5.0
	 *
	 * @param WP_User|WP_Error $user     WP_User or WP_Error object if a previous
	 *                                   callback failed authentication.
	 * @param string           $password Password to check against the user.
	 */
	$user = apply_filters( 'wp_authenticate_user', $user, $password );
	if ( is_wp_error( $user ) ) {
		return $user;
	}

	if ( ! wp_check_password( $password, $user->user_pass, $user->ID ) ) {
		return new WP_Error(
			'incorrect_password',
			sprintf(
				/* translators: %s: User name. */
				__( '<strong>Error</strong>: The password you entered for the username %s is incorrect.' ),
				'<strong>' . $username . '</strong>'
			) .
			' <a href="' . wp_lostpassword_url() . '">' .
			__( 'Lost your password?' ) .
			'</a>'
		);
	}

	return $user;
}