The 401(k) plan, in this case Fidelity, is required to track that for you. Still, 401(k) plans sometimes lose that information, particularly if there are rollovers from one 401(k) to another employer's 401(k) involved, so you'll also want to track that in your own records in case the 401(k) plan ever issues a Form 1099-R that shows a taxable amount that does not correctly reflect your basis.
