How can information found in history books such as "Al-Bidaya wa Al-Nihaya" be authenticated, even though the chain of transmission (isnad) is not always mentioned?

The narrations used to prove a legal ruling, or to support the soundness of a conduct, must have their chain of transmission (isnad) and text (matn) studied. If they are not supported by an isnad, then it is obligatory to extract what needs to be authenticated from books with isnads to verify their authenticity. Otherwise, they cannot be used as proof. The general Muslim public may emulate trustworthy scholars in their judgment regarding the authenticity of narrations and chains of transmission.