The Book of Oaths and Vows
35/39. Chapter:
Saying: "If Allah Wills"
Sunan an-Nasai
أَخْبَرَنَا مُحَمَّدُ بْنُ مَنْصُورٍ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ أَيُّوبَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " مَنْ حَلَفَ فَقَالَ إِنْ شَاءَ اللَّهُ فَقَدِ اسْتَثْنَى " .
It was narrated that Ibn 'Umar said: "The Messenger of Allah said: 'Whoever swears an oath and says: If Allah wills, then he has made an exception.'"
Reference : Sunan an-Nasa'i 3829In-book reference : Book 35, Hadith 69English translation : Vol. 4, Book 35, Hadith 3860