سنن النسائي

It was narrated from Mus'ab bin Sa'd that his father said: "We were talking about something, and I had only recently left Jahiliyyah behind, so I swore by Al-Lat and Al-'Uzza. The Companions of the Messenger of Allah (ﷺ) said to me: 'What a bad thing you have said! Go to the Messenger of Allah (ﷺ) and tell him, for we think that you have committed Kufr.' So I went to him and told him, and he said to me: 'Say: La ilaha illallah wahdahu la sharika lah (There is none worthy of worship except Allah alone, without partner) three times, and seek refuge with Allah from the Shaitan three times, and spit dryly to your left three times, and do not say that again.'"