Sunan an-Nasa'i, Book 35, Hadith 95 (In-Book) ⓘ
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."'"
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 " .
also referenced under: Hadith 3855 (Reference) & Volume 4, Book 35, Hadith 3886 (DarusSalam)