Talmud for Gittin 4:1
הַשּׁוֹלֵחַ גֵּט לְאִשְׁתּוֹ וְהִגִּיעַ בַּשָּׁלִיחַ, אוֹ שֶׁשָּׁלַח אַחֲרָיו שָׁלִיחַ וְאָמַר לוֹ, גֵּט שֶׁנָּתַתִּי לְךָ בָּטֵל הוּא, הֲרֵי זֶה בָטֵל. קָדַם אֵצֶל אִשְׁתּוֹ אוֹ שֶׁשָּׁלַח אֶצְלָהּ שָׁלִיחַ וְאָמַר לָהּ, גֵּט שֶׁשָּׁלַחְתִּי לִיךְ בָּטֵל הוּא, הֲרֵי זֶה בָטֵל. אִם מִשֶּׁהִגִּיעַ גֵּט לְיָדָהּ, שׁוּב אֵינוֹ יָכוֹל לְבַטְּלוֹ:
If one sent a get to his wife, and he came upon the messenger, or if he sent a messenger after him and said to him: "The get that I gave you is void," it is void. [("if he came upon the messenger":) without having pursued him to overtake him, but the messenger stopping on the way and the husband chancing upon him and voiding the get. Even so, it is void, and we do not say that he was simply taunting him and that if he really had wanted to void it, he would have pursued him.] If he reached his wife first or sent a messenger to her, telling her: "The get that I sent you is void," it is void. Once the get has reached her hand (however) he can no longer void it. [We are hereby apprised that even though we see him bent upon voiding it, we do not say that this is clear indication of its having been voided. And in an instance in which a man gives his wife a get (to take effect) at a certain time or with (the fulfillment of) a certain condition — if he says to her: "This is your get from now, (to take effect) at that time," or "if that condition is fulfilled," once the get reaches her hand, he can no longer void it, and she is divorced at that time or with the fulfillment of that condition. And if he did not say to her: "from now," then even after the get reached her hand, he can void it.]