"The principle of "Be conservative in what you send, be liberal in what you accept" is often referred to as the Robustness Principle, also known as Postel's Law, named after Jon Postel, who wrote it in the context of TCP in 1981.
The statement can be found in Request for Comments (RFC) 793, which is the specification for the Transmission Control Protocol (TCP), in the section on "Robustness Principle". The principle was intended to guide the design of network protocols and was intended to encourage interoperability between different systems by suggesting that protocol implementations should be liberal in what they accept (i.e., they should accept a wide range of inputs) and conservative in what they send (i.e., they should limit their output to what is necessary and known to be correct)."