"The two car-bombs were found near Riyadh for apparent use in attacks," the source said, specifying they were in the region of Rumah, 100km away.
The latest discovery brought to five the number of car bombs seized in Saudi Arabia within the past week.
An interior ministry official announced on Sunday the arrest of eight suspects linked to recent deadly clashes between armed fighters and security forces and the booby-trapping of cars.
Security forces had also seized three vehicles packed with thousands of kilograms of explosives, including one they had been searching for since February, the official said, quoted by the state SPA news agency.
A series of human bombings targeting residential compounds killed 52 people in Riyadh in May and November 2003.
The attacks were blamed on suspected sympathisers of Saudi-born Usama bin Ladin's al-Qaida network.
On Thursday, the United States ordered non-essential diplomats in Saudi Arabia to leave the country and urged US citizens living in the kingdom to depart, citing "credible information indicating that extremists are planning further attacks against US and Western interests".
On 12 April, a member of the security forces was killed and a man gunned down during a clash in an eastern neighborhood of Riyadh.
The next day, four policemen were killed by machine-gun fire in separate attacks on the road linking Riyadh and Qasim, northwest of the capital, and authorities said they had discovered two explosives-laden cars.