What did the Prophet Muhammad (saw) instruct `Umar bin Al-Khattab to do when he noticed him swearing by his father?
 Narrated Ibn `Umar:                     Allah's Apostle met `Umar bin Al-Khattab while the latter was going with a group of camel-riders, and he was swearing by his father. The Prophet said, "Lo! Allah forbids you to swear by your fathers, so whoever has to take an oath, he should swear by Allah or keep quiet."