expression
Used to cite a source of information, typically a person, report, or data. Often followed by a clause stating what that source says.
See also: によって
天気予報によれば、明日は雨が降るそうです。
According to the weather forecast, it will rain tomorrow.
彼の話によれば、会議は来週に延期されたらしい。
According to him, the meeting has been postponed to next week.
によって is a more general particle meaning 'by means of', 'depending on', or 'due to', while によれば is specifically for citing a source of information.
Derived from the particle に plus the conditional form of the verb よる (拠る・依る・因る), meaning 'to be based on' or 'to depend on'. The exact historical derivation is straightforward.