The Environmental Working Group (EWG) tested a wide variety of oat and wheat products for glyphosate, the most commonly used herbicide in the world. They found glyphosate contamination in a wide variety of foods, including Cheerios.
See here for a CNN article about EWG’s findings.
See here for a some of EWG’s findings, and here for a more complete list of common food products found to contain glyphosate.