The Book of Oaths and Vows
35/43. Chapter:
The Exception (Saying: "If Allah Wills")
Sunan an-Nasai
أَخْبَرَنَا نُوحُ بْنُ حَبِيبٍ، قَالَ أَنْبَأَنَا عَبْدُ الرَّزَّاقِ، قَالَ أَنْبَأَنَا مَعْمَرٌ، عَنِ ابْنِ طَاوُسٍ، عَنْ أَبِيهِ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " مَنْ حَلَفَ عَلَى يَمِينٍ فَقَالَ إِنْ شَاءَ اللَّهُ فَقَدِ اسْتَثْنَى " .
It was narrated that Abu Hurairah 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 3855In-book reference : Book 35, Hadith 95English translation : Vol. 4, Book 35, Hadith 3886