CTE to get all children and nested children of every parent

I'm stuck in a problem that continuously hammering in my mind to find the solution.

 

I have a users table with 6k of rows and they are related as parent-child. The problem is that I want results not only at a single level parent-child relationship but at all levels (until the last child) for every userid.

 

I have this data in my users table.

 

Tbl_Users
UserID	Parent ID
1	0
2	1
3	1
4	2
5	2
6	5
7	6
8	6
9	0

 

I want this data as the output of CTE.

 

Output
Parent ID	UserID
1	1
1	2
1	3
1	4
1	5
1	6
1	7
1	8
2	2
2	4
2	5
2	6
2	7
2	8
3	3
4	4
5	5
5	6
5	7
5	8
6	6
6	7
6	8
7	7
8	8
9	9

 

Note: The ParentID includes not only its children but also children of its children and itself too. I'm using MSSQL 2019.