Ma'rur al-Kalbi narrates from Sayyiduna Umar (may Allah be pleased with him) that he forbade fars in slaughtering. Abu Ubayd says that Abu Ubaydah said: Fars means nakh'; one says: "I did fars to the goat and nakh' to it," that is, when slaughtering, the knife is taken down to the nukha', which is a bone in the neck. It is also said that it is what lies in the vertebrae of the spine, like marrow, and is joined to the nape; that is, he forbade carrying the slaughter down to it. Abu Ubayd says: As for nakh', it is as Abu Ubaydah said, but there is disagreement about fars; it is said to mean breaking, and he only forbade breaking the neck of the slaughtered animal before it becomes cold. What makes this clear is that in the hadith there is: "And do not hasten the souls before they depart."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19135