And where does the constitution say this? What Article, what clause is that under?
Fourteenth Amendment.
Section 1. All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the state wherein they reside. No state shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any state deprive any person of life, liberty, or property, without due process of law; nor deny to
any person within its jurisdiction the equal protection of the laws.
People with green cards are legal residents and therefore entitled to equal protection. National origin has long been held by common law under the Civil Rights Act of 1964 to apply as a basis of possible discrimination. The government can take away an individual's green card for cause, but it can't take away green cards or otherwise deny holders equal protection based on national origin. That is pretty fundamental in the law, and why no federal judge, including all of those on the SCOTUS, would vote to uphold this order.
If Trump pursues this, it will be unanimous in the 9th Circus, and again unanimous against him in the SCOTUS.