מדרש על חולין 269:7

"your flock": and not that of others (i.e., of gentiles [that you have purchased]). From here it was ruled that one who purchases the shearing of a flock belonging to gentiles is exempt from the (obligation of) the first of the shearing. The shearing of his flock (that he purchased) from his neighbor — if the seller left over (of the shearing itself) for himself, the seller is liable (for the first of the shearing); if he did not, (i.e., if he told the buyer: The first of the shearing of the Cohein is with you), the buyer is liable.