Source

File: wp-includes/widgets/class-wp-widget-rss.php

public function form( $instance ) {
		if ( empty( $instance ) ) {
			$instance = array(
				'title'        => '',
				'url'          => '',
				'items'        => 10,
				'error'        => false,
				'show_summary' => 0,
				'show_author'  => 0,
				'show_date'    => 0,
			);
		}
		$instance['number'] = $this->number;

		wp_widget_rss_form( $instance );
	}