Background check and a temporary job

Hi so I was just offered a job, yay! They said everything with the background check needs to clear before it’s official which makes sense. A few months back I worked temporarily for a company but I wasn’t the right fit for the position and I left. I don’t put that job on my resume, but should I include it on the background check? Am I legally obligated to? And will that look suspicious that I didn’t divulge the information sooner? Thanks in advanced.

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