Tosefta for Demai 6:11
אהַמּוֹכֵר פֵּרוֹת בְּסוּרְיָא, וְאָמַר מִשֶּׁל אֶרֶץ יִשְׂרָאֵל הֵן, חַיָּב לְעַשֵּׂר. מְעֻשָּׂרִין הֵן, נֶאֱמָן, שֶׁהַפֶּה שֶׁאָסַר הוּא הַפֶּה שֶׁהִתִּיר. מִשֶּׁלִּי הֵן, חַיָּב לְעַשֵּׂר, מְעֻשָּׂרִין הֵן, נֶאֱמָן, שֶׁהַפֶּה שֶׁאָסַר הוּא הַפֶּה שֶׁהִתִּיר. וְאִם יָדוּעַ שֶׁיֶּשׁ לוֹ שָׂדֶה אֶחָד בְּסוּרְיָא, חַיָּב לְעַשֵּׂר:
One who sells fruit in Syria and says: "These [fruits] are from the Land of Israel", one must tithe [the purchased fruit]. [If the seller says:] "Tithes have been taken from these [fruits]", he is trusted, as the mouth which caused the prohibition [by saying they are from Israel and therefore require tithes] is the mouth which made them permitted [by saying that tithes have already been taken]. [If the seller says:] "They are mine", one must tithe. [If the seller says:] "They have been tithed", he is trusted, as the mouth which caused the prohibition is the mouth which made them permitted. However, if it is known that he [the seller] has a field in Syria, one must tithe.