What is swearing by Allah, how is it done, and what is forbidden in it?

Question:
May Allah reward you, Honorable Shaykh. This questioner says: What is swearing by Allah? How is it done? And what is forbidden in it?

Answer:
Swearing by Allah is to swear by Allah that one will do such and such, or to swear by Allah that Allah will do such and such. If this is in the category of having a good opinion of Allah (Mighty and Majestic), then there is nothing wrong with it. Indeed, among the servants of Allah are those whom, if they were to swear by Allah, He would fulfill.
So if one swears by Allah that He will do good, that He will send down rain, that He will show mercy, that He will accept the repentance of so-and-so, or that He will forgive so-and-so, this is having a good opinion of Allah, and it is permissible. But if the swearing by Allah is that He will not do good, that He will not forgive so-and-so, or if one swears by Allah that He will not cure so-and-so of his illness, then this is forbidden, and there is a severe warning in it. The man who said, "By Allah, Allah will not forgive so-and-so," Allah the Exalted said: "Who is it who swears by Me that I will not forgive so-and-so? I have indeed forgiven him, and I have rendered your deed void." So if this is in the category of having a bad opinion of Allah, and in the category of preventing good—preventing Allah from bestowing His bounty upon whom He wills of His servants—then this is forbidden, and there is a severe warning in it. Yes.