I was able to find the letter written from Beauregard to Anderson regarding the fort's immediate evacuation.
APRIL 11TH, 1861: Gen. Beauregard wrote to Major Anderson that his government had been patiently awaiting word from the U.S. government that the fort would be evacuated, but it hadn't come: "But the Confederate States can no longer delay assuming actual possession of a fortification commanding the entrance to one of our harbors, and necessary to its defense and security.
"I am ordered by the Government of the Confederate States to demand the evacuation of Fort Sumter."
Beauregard was sending his aides Col. James Chesnut and Capt. Stephen D. Lee to formally make the demand. They will assist in the removal of yourself and command with arms and property and all private property to any post in the U.S. you want. Furthermore, Anderson would be allowed to salute the flag when taking it down.
The Refusal of This Demand Was the Final Straw, Especially With the Arrival of the Harriet Lane Later That Day. The Expected Reinforcements Were Arriving. --Old B-Runner