Tosefta for Chullin 227:46

An udder cooked with its milk is permitted. A stomach cooked [along with the undigested milk inside it] is liable. One who cooks meat with milk, behold, he is liable. How much must one cook to be liable? Half an olive's bulk of meat and half an olive's bulk of milk, in order that it adds up to an olive's bulk. In the same way that one is liable for cooking, one is liable for eating. How much must one eat to be liable? It suffices that he have eaten any amount of cooked food [containing a forbidden mixture of meat and milk]. One who cooks [meat] in whey is exempt (Hul. 114a:4); in the milk of a male [animal] is exempt (Hul. 113b:7). Blood cooked in milk is exempt. The bones and the sinews and the horns and the hooves the are cooked in milk are exempt. [A Kohen who] cooks piggul (a sacrifice that becomes unfit due to the Kohen's intention to consume it after its prescribed time) or notar (a sacrifice not consumed within its prescribed time) or an impure [animal] in milk is liable due to their being piggul, notar, or impure (which are all more serious violations than cooking meat with milk). [Ritually] pure meat from a domesticated animal [cooked] with milk of a [ritually] pure animal is forbidden for [the purpose of] healing or to derive benefit. Rabbi Shimon permits it for [the purpose of] deriving benefit. The meat of a pure domesticated animal [cooked in] the milk of an impure wild animal (such as a deer), [or] the meat of a pure wild animal or a [pure] fowl [cooked in] the milk of an impure domesticated animal -- Rabbi Akiva exempts the cooked dish, as it is said, "A kid," "a kid," three times (Ex. 23:19, Ex. 34:26, Deut. 14:21, see Hul. 8:4).