Sunan Ibn Majah, Book 11, Hadith 2184 (In-Book) ⓘ
It was narrated from Ibn 'Umar: "Whoever swears an oath and says In sha' Allah, will never break his oath."
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مُحَمَّدٍ الزُّهْرِيُّ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنْ أَيُّوبَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، رِوَايَةً قَالَ " مَنْ حَلَفَ وَاسْتَثْنَى فَلَمْ يَحْنَثْ " .
also referenced under: Hadith 2106 (Reference) & Volume 3, Book 11, Hadith 2106 (DarusSalam)