I seem to recall reading (within the last year or so) that an aircraft's CofA is invalidated following refuelling until a fuel sample has been taken and checked.
If by "an aircraft" you mean a specific type, I cannot contradict that. If you mean any aircraft, that is certainly incorrect. There is no such general requirement.
The commander is required to ensure
that any pre-flight check system established by the operator and set forth in the operations manual or elsewhere has been complied with by each member of the crew of the aircraft.
If that pre-flight check system includes a sampling after refuelling, then it must be carried out.