Sunan Ibn Majah
2105
Sahih+3

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

SahihAl-Albani
SahihMuhammad Fouad Abd al-Baqi