Source

File: wp-includes/post.php

function set_post_thumbnail( $post, $thumbnail_id ) {
	$post         = get_post( $post );
	$thumbnail_id = absint( $thumbnail_id );
	if ( $post && $thumbnail_id && get_post( $thumbnail_id ) ) {
		if ( wp_get_attachment_image( $thumbnail_id, 'thumbnail' ) ) {
			return update_post_meta( $post->ID, '_thumbnail_id', $thumbnail_id );
		} else {
			return delete_post_meta( $post->ID, '_thumbnail_id' );
		}
	}
	return false;
}