Gittin, Chapter 7, Mishnah 2. This mishnah deals with a man who fell ill or was about to set out on a long journey and the like, and people - or the witnesses - came and asked him: "Nichtov get l'ishtecha?" - "Shall we write a get for your wife?", so that if he dies his wife will not be left waiting for a yavam. And he answered them: "Kitvu" - "Write it."
What actually happened:
Instead of writing the get themselves, they appointed others to do so:
Telling the scribe: They told the scribe to write the get, and he wrote it.
Telling the witnesses: They told the witnesses to sign, and they signed.
After the get was written and signed, they brought it back to the husband. The husband took this get - written and signed by people other than those he had asked - and handed it to his wife himself. Even so, "harei ha'get batel" - the get is void.
The essential condition:
The get is valid only "ad she'yomar la'sofer: k'tov, v'la'edim: chotomu" - until he himself says to the scribe, "Write," and to the witnesses, "Sign" - the instruction must come from the husband directly to the scribe and to the witnesses, and not through a third party. And even though the get was returned to his hands and he gave it to her himself, that does not remove the requirement that the husband speak to them directly.
This mishnah follows the view of Rabbi Yosei that we learned in the previous chapter: even a beis din that the husband appointed to write the get cannot pass these instructions on to someone else; the matter must be carried out by them themselves.
In summary: In this mishnah we learned that a man who was asked whether a get should be written for his wife and answered "Write it," and instead of writing it themselves they told a scribe and he wrote it and told witnesses and they signed - the get is void, even if it was returned to the husband and he himself gave it to his wife, since a direct instruction from the husband to the scribe and to the witnesses is required, in accordance with the view of Rabbi Yosei that these instructions may not be passed on to another.