(39) باب الاِسْتِثْنَاءِ .
It was narrated from Ibn 'Umar that the Messenger of Allah said:
"Whoever swears an oath and says, 'If Allah wills,' then he has the choice: If he wishes, he may go ahead, and if he wishes he may not."
Reference: Sunan an-Nasa'i 3830
In-book reference: Book 35, Hadith 70
English translation: Vol. 4, Book 35, Hadith 3861