https://en.wikipedia.org/wiki/Finances_of_the_Islamic_State

Website The Daily Beast in June 2014 accused wealthy donors in Saudi Arabia and Qatar of having funded IS in the past.[45][46] Iran and Iraqi Prime Minister Nouri al-Maliki have accused the governments of Saudi Arabia and Qatar of funding the group.[47][45][48] Ahead of the pro-Iraq, anti-IS conference held in Paris on 15 September 2014, France’s foreign minister acknowledged that a number of countries at the table had “very probably” financed IS’s advances.[49] According to The Atlantic, IS may have been a major part of Saudi Arabian Bandar bin Sultan’s covert-ops strategy in Syria.[50]

See https://www.theatlantic.com/international/archive/2014/06/isis-saudi-arabia-iraq-syria-bandar/373181/ for the Citation 50

Also see https://www.channel4.com/news/factcheck/factcheck-qa-is-saudi-arabia-funding-isis for an overview of various US internal documents where the US government itself accused Saudi Arabia of funding Daesh at least in the beginning.

Perhaps the most powerful indication of Saudi’s financial links with ISIS can be seen in the cache of emails leaked from the office of Hillary Clinton, who was US Secretary of State from 2009 to 2013.

The messages, published by Wikileaks, contain an unambiguous statement by her campaign chairman, John Podesta:

“We need to use our diplomatic and more traditional intelligence assets to bring pressure on the governments of Qatar and Saudi Arabia, which are providing clandestine financial and logistic support to ISIL and other radical Sunni groups in the region.”

This wasn’t the first time US officials had made this claim. In 2009, Wikileaks published diplomatic cables from the US State Department which spelt out the same concerns.

“Donors in Saudi Arabia constitute the most significant source of funding to Sunni terrorist groups worldwide,” the documents said. “While the Kingdom of Saudi Arabia (KSA) takes seriously the threat of terrorism within Saudi Arabia, it has been an ongoing challenge to persuade Saudi officials to treat terrorist financing emanating from Saudi Arabia as a strategic priority …

“More needs to be done since Saudi Arabia remains a critical financial support base for al-Qaeda, the Taliban, LeT, and other terrorist groups, including Hamas, which probably raise millions of dollars annually from Saudi sources.”