Hi, My 6 Years Old Boy Started Putting His Finger

Hi, My 6 years old boy started putting his finger in is butt hole (specially drink bath time) and when i asked him nicely why he is doing that his answer was he is looking for poop … I really don't know what's the real reason he is doing that and I don't know if i should worry about it and what to do about this satiation .Please help.