Chapter: How to swear?

A man came to Allah's Messenger (ﷺ) asking him about Islam, Allah's Messenger (ﷺ) said, "You have to offer five compulsory prayers in a day and a night (24 hours)." The man asked, "Is there any more compulsory prayers for me?" Allah's Messenger (ﷺ) said, "No, unless you like to offer Nawafil (i.e. optional prayers)." Allah's Messenger (ﷺ) then added, "You have to observe fasts during the month of Ramadan." The man said, "Am I to fast any other days?' Allah's Messenger (ﷺ) said, "No, unless you wish to observe the optional fast voluntarily." Then Allah's Messenger (ﷺ) told him about the compulsory Zakat. The man asked, "Do I have to give anything besides?" Allah's Messenger (ﷺ) said, "No, unless you wish to give in charity voluntarily." So, the man departed saying, "By Allah I will neither do more nor less than that." Allah's Messenger (ﷺ) said, "If he has said the truth he will be successful."

The Prophet (ﷺ) said, "Whoever has to take an oath should swear by Allah or remain silent." (i.e. He should not swear by other than Allah.)