ENOfficial
It was narrated from Ibn 'Umar that the Messenger of Allah (ﷺ) said: "Whoever swears an oath and says In sha' Allah, if he wishes he may go ahead and if he wishes he may not, without having broken his oath."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُحَمَّدُ بْنُ زِيَادٍ، حَدَّثَنَا عَبْدُ الْوَارِثِ بْنُ سَعِيدٍ، عَنْ أَيُّوبَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ " مَنْ حَلَفَ وَاسْتَثْنَى إِنْ شَاءَ رَجَعَ وَإِنْ شَاءَ تَرَكَ غَيْرُ حَانِثٍ " .
sunnah.com