How to use the /testfor command to detect what a player says in chat [duplicate]

How do I use the /testfor command in a command block to test for a specific word that a Minecraft player playing on the server says. For my example, if a player says:

[Player] - Hi

The command block then says:

[Command Block] - Hello.

Please help, just trying to create a cool command =) {If This Works Please Tell Me}