Where did the belief about monks' mothers going to heaven come from?

Some people believe that in Buddhism when the mother of a monk dies she would hold on to the monk's robe and go to heaven. I never read the Buddha say anything like that, but of course I haven't read everything the blessed one said. It makes me wonder because in Buddhism there are no shortcuts and it may look like one. I've heard this belief exists in Thailand, does anyone have more information on that, like did the Buddha ever suggested it?