Render shadow without background!!!

I am trying to render an objects with shadow. But I don’t want the background, background is a plane object. I only want the object and its shadow then save it as png with alpha channel.

Thanks in advance

Give the plane a material and in the Material / Shadow options enable ‘Shadows Only’
When you render set .png as the output file format and set RGBA

Did not thought it works like that. Thank you.