Saurapurana 9.25

9.25

āgneyaṃ pratipadyeva pradadyādāhitāgnaye
rājasūyasya yajñasya phalaṃ bhavati śāśvatam

If one gives the Āgneya on the very first [lunar day] to a maintainer of the sacred fire, / the fruit becomes the everlasting fruit of a Rājasūya sacrifice.