INSERT statement

The customer_id is INT type.

Which one is correct between following two INSERT statement? (difference is only customer_id value 1 and '1' in the VALUES(.....))

INSERT INTO customer 
(customer_id, first_name, last_name, employed_date) 
VALUES (1, 'Tom', 'Jones', '08/22/2014')

and 

INSERT INTO customer 
(customer_id, first_name, last_name, employed_date) 
VALUES ('1', 'Tom', 'Jones', '08/22/2014')