Washington (CNN) Workers at the Pennsylvania petrochemical plant where President Donald Trump spoke Tuesday were told that if they didn't attend the event, they either had to use paid time off or receive no pay for the day.

At least some of the workers who attended the speech were instructed not to protest the President, who told the crowd of workers at the Royal Dutch Shell plant he would be imploring their union leaders to support his reelection.

The instructions to the workers came in a memo, a copy of which was obtained by CNN's Polo Sandoval from a congressional source. That source was given the memo by a person in Beaver County, Pennsylvania -- the site of the plant.

"Your attendance is not mandatory. This will be considered an excused absence. However, those who are NOT in attendance will not receive overtime pay on Friday," read part of the memo.

Shell spokesman Curtis Smith confirmed workers were told they would also miss out on some overtime pay if they skipped the event. Shell said it did not write the memo.

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