The Talmud not only mentions signatures; it has rules for them.
In ancient times, a signature was required for all kinds of economic transactions. If you wanted to buy a goat, you had to sign a document. And just like with credit cards today, sometimes you had to sign even for smaller purchases.
Source: Why Do We Sign For Things? A Rabbi, A Lawyer And A MasterCard Exec Explain : Planet Money : NPR
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