Sunan Ibn Majah 2106
سنن ابن ماجه#2106Sahihحَدَّثَنَا 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
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
Al-AlbaniSahih
Muhammad Fouad Abd al-BaqiSahih
Shuaib Al ArnautSahih
Zubair Ali ZaiSahih
In-book reference: 11:17