The Book of Oaths and Vows
35/18. Chapter:
Whoever Swears An Oath And Says: "If Allah Wills"
Sunan an-Nasai
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."
Reference : Sunan an-Nasa'i 3793In-book reference : Book 35, Hadith 33English translation : Vol. 4, Book 35, Hadith 3824