What did the Prophet Muhammad (peace be upon him) say when he saw `Umar bin Al-Khattab swear by his father?
 Narrated Ibn `Umar:                     that he found `Umar bin Al-Khattab in a group of people and he was swearing by his father. So Allah's Apostle called them, saying, "Verily! Allah forbids you to swear by your fathers. If one has to take an oath, he should swear by Allah or otherwise keep quiet."