That the Messenger of Allah ﷺ said: "Whoever swears [about an oath] and says: 'If Allah wills (Insha Allah), then he will not have broken it." (Using translation from Tirmidhī 1532)
عَنْ رَسُولِ اللَّهِ ﷺ قَالَ مَنْ حَلَفَ عَلَى يَمِينٍ ثُمَّ قَالَ إِنْ شَاءَ اللَّهُ فَإِنَّ لَهُ ثُنْيَاهُ