It’s in the FAR’s. 121.436

(3) If serving as pilot in command in part 121 operations, has 1,000 hours as second in command in operations under this part, pilot in command in operations under §91.1053(a)(2)(i) of this chapter, pilot in command in operations under §135.243(a)(1) of this chapter, or any combination thereof. For those pilots who are employed as pilot in command in part 121 operations on July 31, 2013, compliance with the requirements of this paragraph (a)(3) is not required.

And;

(c) For the purpose of satisfying the flight hour requirement in paragraph (a)(3) of this section, a pilot may credit 500 hours of military flight time obtained as pilot in command of a multiengine turbinepowered, fixed-wing airplane in an operation requiring more than one pilot.