Assertion: The rate of a reaction depends on the concentration of reactants(s) according to the rate law expression determined experimentally.

Reason: The rate of reaction decreases as the reaction moves in the forward direction because the concentration of reactant(s) decreases. 

Assertion and Reason both are correct statements and reason is the correct explanation for assertion.

For any reaction, A → B

r = k [A]^x 

where x is the order of a reaction.

As the reaction will proceed, rate of reaction decreases because concentration of reactant decreases as more product will be formed.