Yahya bin Abi Kathir يحيى بن أبي كثير الطائي · The Book of Oaths and Vows

"The oath by which the Messenger of Allah used to swear was: 'No, by the Controller of the hearts.'"

"A vow does not bring anything to the son of Adam that has not been decreed for him. It is just a means of taking wealth from the miserly."

"Sulaiman bin Dawud said: 'Tonight I will go around ninety women, each of whom will bear a horseman who will perform Jihad in the cause of Allah.' His companion said to him: 'If Allah wills.' But he did not say: 'If Allah wills.' Then he went around to them all, but none of them got pregnant except a woman who bore half a man. By the One in Whose Hand is my soul! If he had said, 'If Allah wills,'…

"The expiation for vows is the expiation for an oath."

"The Messenger of Allah said: 'There is no vow to commit an act of disobedience.'"

"There is no vow to commit an act of disobedience and its expiation is the expiation for an oath."