فَقُلْنَا اضْرِبُوهُ بِبَعْضِهَا ۚ كَذَٰلِكَ يُحْيِي اللَّهُ الْمَوْتَىٰ وَيُرِيكُمْ آيَاتِهِ لَعَلَّكُمْ تَعْقِلُونَ
Faqulnad riboohu biba'dihaa kazaalika yuhyil laahul mawtaa wa yureekum aayaatihee la'allakum ta'qiloon
So we said, "Strike him with part of it." In this way God brings the dead to life and shows you his signs, so that you may understand.
The Qur'an does not say which part of the animal was used, and commentators who supplied an answer acknowledged they were doing so from transmitted reports rather than the text. What the verse states plainly is the purpose, which is a demonstration of raising the dead, so the cow sequence turns out to be about the resurrection argument that this surah keeps returning to. Understanding is what it asks for at the close.