Relying on just one source for a fact can be problematic. Any one source can be incorrect. One has to consider how reliable that source probably is and know knowledgeable the likely informant was. Sometimes those two things can be difficult to determine.
While more that one source for the same fact is ideal, it’s not always possible to get multiple sources–particularly independent ones. But don’t find one source and immediately think you’ve answered the question. You might have.
But you might not have either.