add_filter( 'the_content', function( $c ) {

	if( !is_single( ) ) {
		return $c;
	}

	global $post;

	$src = get_post_custom_values( 'source_field', $post->ID );

	if ( is_array( $src ) && isset( $src[0] ) ) {
		$src = $src[0];
	} else {
		$src = '';
	}

	if( $src ) {
		return $c . '<p class="source"><span>Source: </span>' .  $src . '</p>';
	} else {
		return $c;
	}


}, 5, 1 );