Yoreh Deah Siman 16 Seif 11

 

יא  עוֹבֵד כּוֹכָבִים שֶׁמָּכַר שְׁתֵּי בְּהֵמוֹת וְאַחַר כָּךְ אָמַר, מֵסִיחַ לְפִי תֻּמּוֹ, שֶׁהֵן אוֹתוֹ וְאֶת בְּנוֹ, אֵינוֹ נֶאֱמָן בַּמֶּה שֶּׁאָמַר לְאַחַר שֶׁמְּכָרָם וְיָצְאוּ מִתַּחַת יָדוֹ; וּמִיהוּ אִי מְהֵימָן לֵיהּ, אָסוּר.

 

Seif 11

An idolater that sells two animals and who then says,  I have been told that these are mother and child, we do not believe what they say after the sale is finalized and the animals are out of his possession, and if the gentile is (known to be) trustworthy then it is forbidden (to slaughter on the same day).