Musnad Ahmad 1589

It was narrated from Mus"ab bin Sa"d that his father said: I swore an oath by al-Lat and al-"Uzza, and my companions said: You have said something unseemly. So I went to the Prophet (ﷺ) and said: I am newly Muslim, and I swore an oath by al-Lat and al-"Uzza. The Messenger of Allah (ﷺ) said: "Say La ilaha illallah wahdahu (there is no god but Allah alone) three times, and spit dryly to your left three times, and seek refuge with Him, and do not do it again."