This hadith is about following the teachings and instructions given by Prophet Muhammad (PBUH). It was narrated by 'AbdurRahman bin 'Awf that Umar bin al-Khattab wanted to deliver a speech to people but he was advised to delay it until he reached Madinah. When Umar reached Madinah, AbdurRahman heard him saying that some people were asking why should we stone adulterers when in the book of Allah only flogging is mentioned. He further said that although this is not mentioned in the book, but since Messenger of Allah stoned adulterers and we stoned them after him, if it wasn't for people thinking that something has been added which isn't part of it, he would have inserted it as revealed. This hadith teaches us about following what has been taught by our beloved prophet without adding or subtracting anything from his teachings. We must always remember that whatever our beloved prophet taught us was revealed upon him by Almighty Allah and therefore must be followed without any doubt or hesitation.