"The Messenger of Allah said: 'Whoever swears an oath and says: "If Allah wills, then he has made an exception."'"
Sunan an-Nasa'i Book 35, Hadith 95
← Previous Hadith "The Messenger of Allah came to a man who was being supported by two others and said: 'What is the matter with him?' It was said: 'He vowed to walk to the Ka'bah.' He said: 'Allah does not benefit from his torturing himself.' And he told him to ride."

Sunan an-Nasa'i Book 35, Hadith 94 Next Hadith → "Sulaiman said: 'I will certainly go around to ninety women tonight, each of whom will bear a child who will fight in the cause of Allah.' It was said to him: 'Say: If Allah wills' but he did not say it. He went around to them but none of them bore a child except for one woman who bore half a person." The Messenger of Allah said: "If he had said: 'If Allah wills,' he would not have broken his vow, and this would have been a means to help him to get what he wanted."

Sunan an-Nasa'i Book 35, Hadith 96