Open sources/classes/post/class_post_edit.php and find:

//-----------------------------------------
// Reason for edit?
//-----------------------------------------
if ( $this->moderator['edit_post'] OR $this->ipsclass->member['g_is_supmod'] )
{
$this->post['post_edit_reason'] = trim( $this->ipsclass->input['post_edit_reason'] );
}
Change to: //-----------------------------------------
// Reason for edit?
//-----------------------------------------
if ( $this->moderator['edit_post'] OR $this->ipsclass->member['g_is_supmod'] )
{
if ( $_POST['post_edit_reason'] == "" )
{
$this->ipsclass->Error( array( LEVEL => 1, MSG => 'reason_for_edit') );
}
else
{
$this->post['post_edit_reason'] = trim( $this->ipsclass->input['post_edit_reason'] );
}
}
Open cache/lang_cache/en/lang_error.php and find );. Add above:'reason_for_edit' => "You must enter a reason for editing this post!",When a mod edit a post and not enter a reason for it, will show the this messag:














