וַיִּזְרַ֤ע יִצְחָק֙ בָּאָ֣רֶץ הַהִ֔וא וַיִּמְצָ֛א בַּשָּׁנָ֥ה הַהִ֖וא מֵאָ֣ה שְׁעָרִ֑ים וַֽיְבָרֲכֵ֖הוּ יְהוָֽה׃
And Isaac sowed in that land, and found in the same year a hundred-fold; and the LORD blessed him.
Rashi on Genesis
בארץ ההוא IN THAT LAND although it (Gerar) had not the sacred importance of Erez Yisroel proper — i. e., as the territory of the seven nations, and is therefore not as fertile as the Holy Land (cf. Ketubot 112a).
Rashbam on Genesis
בארץ ההיא, even though the soil of this land is exceptionally hard;
Sforno on Genesis
ויזרע יצחק בארץ ההיא, in accordance with G’d’s instructions “גור בארץ הזאת” (verse 3)