Return flight needed to be purchased?

I just got my UK Visa (youth mobility scheme) in less that one week!! yay!! Now I've got a question regarding flights. I heard somewhere that you need to purchase a return flight to get through immigration. Is this true? I was going to buy a one way ticket, as my visa is for two years and I'm not sure of when I will be returning, but I heard I need to show proof that I have a return flight? Does anyone know if this is true or not??
Thank you in advance!!

This isn't true, I never had one, nor did any of my other Canadian friends living here in the UK and it was fine.