Abu Hurairah (May Allah be pleased with him) said:
The Prophet (ﷺ) said, "He who takes an oath and involuntarily says: 'By Al-Lat and Al-'Uzza' should at once affirm: 'La ilaha illallah (there is no true god except Allah)', and he who says to his companion: 'Come let's gamble' should make expiation by giving something in charity."[Al-Bukhari and Muslim].
App reference: Book 18, Hadith 297
Arabic/English book reference: Book 18, Hadith 1807