The Prophet (peace and blessings be upon him) said:
"Whoever swears an oath and says 'In sha Allah (if Allah wills)', then if he wishes he may carry it out, and if he wishes he may leave it, and he will not bear the sin of breaking his oath."
Hadith Referenceصحیح الجامع الصغیر / حرف الميم / 6206
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[ن هـ] عن ابن عمر. الإرواء ٢٥٧٠، ٢٥٧١.