זכר תמים… יקריב אתו לרצונו לפני ה’ (ויקרא א:ג)

The Medrash Rabba relates that when someone brought a bull for a Korben to the Beis Hamikdash,  if however much they prodded the bull to come, it didn’t let itself be pulled – after sacrificing the Korben they would see that the bull wasn’t kosher.  This is what the pasuk alluded to “זכר תמים” – the best sign to see if a bull’s failure renders it unkosher is to “יקרוב אתו….” – if the bull lets itself to be led or not.