The Messenger of Allah (PBUH) caught 'Umar while he going in a caravan, swearing by his father. The Messenger of Allah (PBUH) said: Lo! Allah forbids you to swear by your fathers. So whoever swears, let him swear by Allah or remain silent.

A Jew came to the Prophet (PBUH), so he (PBUH) said: You make up rivals to Allah, and you associate partners, you say: If Allah wills and I will, and you say: By the Ka'bah. So the Prophet )PBUH) commanded them; if they wanted to take an oath, to say: By the Lord of the Ka'bah, and for one of them to say: If Allah wills then I will.