As a general rule, when in doubt I always will. You’re pouring new concrete to older, more cured concrete that is considerably harder, so they will expand and contract at different rates which will very likely cause a crack to open between them. If you’re tech-minded, here’s what the American Concrete Institure has to say: https://www.concrete.org/Portals/0/Files/PDF/302.1R-15_Chapter5.pdf