Midrash for Bikkurim 1:4
אֵלּוּ מְבִיאִין וְלֹא קוֹרִין, הַגֵּר מֵבִיא וְאֵינוֹ קוֹרֵא, שֶׁאֵינוֹ יָכוֹל לוֹמַר אֲשֶׁר נִשְׁבַּע ה' לַאֲבוֹתֵינוּ לָתֵת לָנוּ (דברים כו). וְאִם הָיְתָה אִמּוֹ מִיִּשְׂרָאֵל, מֵבִיא וְקוֹרֵא. וּכְשֶׁהוּא מִתְפַּלֵּל בֵּינוֹ לְבֵין עַצְמוֹ, אוֹמֵר, אֱלֹהֵי אֲבוֹת יִשְׂרָאֵל. וּכְשֶׁהוּא בְבֵית הַכְּנֶסֶת, אוֹמֵר, אֱלֹהֵי אֲבוֹתֵיכֶם. וְאִם הָיְתָה אִמּוֹ מִיִּשְׂרָאֵל, אוֹמֵר, אֱלֹהֵי אֲבוֹתֵינוּ:
These people bring, but do not recite: the convert brings but does not recite, for he is unable to say [in the Biblical recitation] "[the land] that God swore to our fathers to give to us." And if his mother was from Israel, he brings and recites. And when he prays by himself, he says [in place of, "the God of our fathers"] "the God of the fathers of Israel." And when he is in the synagogue, he says "the God of your fathers." And if his mother was from Israel, he says "the God of our fathers."