Anong sabi mo

Hi! I'm a beginner in Tagalog so my question is probably going to be very basic. I learned that "Anong sabi mo?" means "What did you say?". But I don't understand... isn't "sabi" just the root? Shouldn't it be conjugated in the past? Like "Anong sinabi mo"??

Can someone help me understand?

Maraming salamat sa inyong lahat!