Prove that you paid the rent on time by presenting the landlord with a copy of the receipt that he or she gave you when you paid cash. Or you could provide the landlord with a copy of the money order you made out to h/h for the rent. Someone once said on the board, however, that all that proved was that you bought a money order.
The burden is on you to prove you paid on time. Paying late multiple times could be considered a breach of contract if that's what your lease or rental agreement says.