Talmud for Gittin 7:6
הֲרֵי זֶה גִטֵּךְ עַל מְנָת שֶׁתְּשַׁמְּשִׁי אֶת אַבָּא, עַל מְנָת שֶׁתֵּנִיקִי אֶת בְּנִי, כַּמָּה הִיא מֵנִיקָתוֹ, שְׁתֵּי שָׁנִים. רַבִּי יְהוּדָה אוֹמֵר, שְׁמֹנָה עָשָׂר חֹדֶשׁ. מֵת הַבֵּן אוֹ שֶׁמֵּת הָאָב, הֲרֵי זֶה גֵט. הֲרֵי זֶה גִטֵּךְ עַל מְנָת שֶׁתְּשַׁמְּשִׁי אֶת אַבָּא שְׁתֵּי שָׁנִים, עַל מְנָת שֶׁתֵּנִיקִי אֶת בְּנִי שְׁתֵּי שָׁנִים, מֵת הַבֵּן אוֹ שֶׁמֵּת הָאָב אוֹ שֶׁאָמַר הָאָב אִי אֶפְשִׁי שֶׁתְּשַׁמְּשֵׁנִי, שֶׁלֹּא בְהַקְפָּדָה, אֵינוֹ גֵט. רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל אוֹמֵר, כָּזֶה גֵט. כְּלָל אָמַר רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל, כָּל עַכָּבָה שֶׁאֵינָהּ הֵימֶנָּה, הֲרֵי זֶה גֵט:
(If he said:) "This is your get on condition that you serve my father," "on condition that you nurse my son," [The gemara explains that where he did not qualify his words, not stating specifically how much time she should serve his father, he is to be understood as having stated "for one day," and the condition is fulfilled if she serves his father or nurses his son one day alone.] …How long does she nurse him? [That is, how long is the time of nursing, within which, if she nurses him for one day, the condition is fulfilled?] Two years. R. Yehudah says: Eighteen months. [But if she nursed him after two years, according to the rabbis, or after eighteen months, according to R. Yehudah, this is not nursing, and the condition has not been fulfilled. The halachah is not in accordance with R. Yehudah.] — if the son died, [and she did not nurse him at all]; or if the father died, [and she did not serve him], it is a get. [For he (the husband) did not wish to taunt her, but only to gain some benefit, and it was not required. Had he known that his father or his son would die, he would not have made the condition in the first place.] (If he said:) "This is your get on condition that you serve my father for two years," "on condition that you nurse my son for two years" — if the son died, or if the father died, or if the father said: "I do not want you to serve me," without offense [i.e., even though she did not anger him, so that the forestalling (of the fulfillment of the condition) is not due to her], it is not a get [and, it goes without saying, if it were with offense.] R. Shimon b. Gamliel said: In such an instance, it is a get, [since she did not offend and was not the cause of the forestalling.] R. Shimon b. Gamliel stated a general rule: (In the instance of) any forestalling which is not due to her, it is a get. [The halachah is not in accordance with R. Shimon b. Gamliel.]