The rate of a reaction can be defined as the change in concentration of a reactant or product in unit time.

The rate of a reaction decreases with the passage of time.

With the continuation of the reaction, the concentration of a reactant decreases with time.

The rate of a reaction is directly proportional to the concentration of reactant so, if the concentration of a reactant is decreasing, then the rate of reaction will also decrease with time.

As an example take equation

(NEGATIVE SIGN SINCE THE RATE IS GETTING NEGATIVE AS REACTANTS ARE DECREASING ON PASSAGE OF TIME).