Source and Reason for Bishul Akum
Eng is R. Brofsky
The gemara (Avoda Zara 37B - 40A), after attempting to attribute the prohibition of bishul akum to a biblical source, concludes that bishul akum is prohibited "mi-derabbanan ve-kra esmachta be-alma" (the prohibition was legislated by the rabbis, but they made a textual connection in the Torah to support their enactment in the Torah)...
MISHNAH. THE FOLLOWING ARTICLES OF HEATHENS ARE PROHIBITED BUT THE PROHIBITION DOES NOT EXTEND TO ALL USE OF THEM:13 MILK WHICH A HEATHEN MILKED WITHOUT AN ISRAELITE WATCHING HIM, THEIR BREAD...