May 15 1862, Wrote Gen. Thomas, “The two Divisions of the Reserve having an extended line to guard with Cavalry, and but about 350 men to do the duty, you will please designate two squadrons from your Command to report to Major Gen. J. A. McClernand, Commdg Reserve, for duty temporarily.”

The Papers of Ulysses S Grant, Vol 5, p 121

National Archives, RG 393, USG Letters Sent.