أَخْبَرَنِي أَحْمَدُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا حِبَّانُ، قَالَ حَدَّثَنَا عَبْدُ الْوَارِثِ، قَالَ حَدَّثَنَا أَيُّوبُ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " مَنْ حَلَفَ فَاسْتَثْنَى فَإِنْ شَاءَ مَضَى وَإِنْ شَاءَ تَرَكَ غَيْرَ حَنِثٍ " .
It was narrated from Ibn 'Umar that the Prophet said: "Whoever swears an oath and says: 'If Allah wills', then if he wishes he may go ahead, and if he wishes he may not, without having broken his oath." Translation Not Available
Book Ref: Sunan An Nisai Book 35 Hadith 3793
Web Ref: Sunan An Nisai Vol 4 Book 35 Hadith 3824