The Chapters on Expiation
11/6. Chapter:
Uttering the exception when swearing
Sunan Ibn Majah
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مُحَمَّدٍ الزُّهْرِيُّ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنْ أَيُّوبَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، رِوَايَةً قَالَ " مَنْ حَلَفَ وَاسْتَثْنَى فَلَمْ يَحْنَثْ " .
It was narrated from Ibn 'Umar: "Whoever swears an oath and says In sha' Allah, will never break his oath."
Reference : Sunan Ibn Majah 2106In-book reference : Book 11, Hadith 17English translation : Vol. 3, Book 11, Hadith 2106