Sunan an-Nasa'i, Volume 4, Book 35, Hadith 3824 (DarusSalam) ⓘ
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."
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 " .
also referenced under: Hadith 3793 (Reference) & Book 35, Hadith 33 (In-Book)