Musnad Ahmad 8:151

Yahya ibn Adam narrated to us, Isra'il narrated to us, from Abu Ishaq, from Mus'ab ibn Sa'd, from his father, who said: I swore by Al-Lat and Al-'Uzza. My companions said: "You have spoken nonsense." So I came to the Prophet, peace and blessings of Allah be upon him, and said: "The covenant (of Islam) was recent, and I swore by Al-Lat and Al-'Uzza." The Messenger of Allah, peace and blessings of Allah be upon him, said: "Say: 'There is no god but Allah alone' three times, then spit to your left three times, seek refuge (in Allah), and do not do it again."