Narrated Ibn 'Umar (RA):
"Allah's Messenger (ﷺ) said: "Whoever swears an oath, and then says: "If Allah Wills", he is not held accountable if he breaks it."[Ahmad and al-Arba'a reported it, and Ibn Hibban graded it Sahih (authentic)]. (DE)
Reference (DE): Bulugh al-Maram 1378 (DE)
In-book reference (DE): Book 13, Hadith 5 (DE)
English translation (DE): Book 13, Hadith 1378 (DE)