As you would expect, it came from an oil company:
“Chevron, the second-largest oil company in the U.S. and eighth-largest in the world, contributed $2.5 million in October to the Congressional Leadership Fund, a super PAC to elect House Republicans. That makes Chevron’s super PAC donation the single largest from a corporation.”
“The donation comes after House Republicans voted 109 times this Congress to enrich oil companies. According to Public Campaign Action Fund’s Adam Smith:"
“The donation appears to be the largest from a publicly-traded corporation in the post-Citizens United era. The corporate donation is double what the company’s PAC and employees have already donated to federal candidates and committees this cycle, according to analysis of data from the Center for Responsive Politics.”
Source: Climate Progress