Musnad Ahmad 284:3

Muhammad ibn Ja‘far narrated to us, he said: Shu‘bah narrated to us, from ‘Ata’ ibn al-Sa’ib, from Abu al-Bakhtari, from ‘Ubaydah, from ‘Abdullah ibn al-Zubayr, from the Prophet, peace and blessings of Allah be upon him: "A man swore by Allah, other than whom there is no god, while lying, and he was forgiven." Shu‘bah said: "Because of the Tawhid (monotheism)."