Andy is correct. Ironically, David Stanley, the chief of cavalry of the Army of the Cumberland, was on medical leave with typhoid fever. Rosecrans requested that he be sent Buford to assume command of his Cavalry Corps. Buford agreed, provided that he could take the Regulars with him when he went west. However, the Army of the Potomac was in the field and Buford was busy, whereas nothing was happening in Chattanooga, so his departure was delayed. By the time he was ready to go, he was down with the typhoid fever that took his life.
Rosecrans' request can be found in the OR. There is a very reliable primary source on Buford's condition and his agreement to go: Capt. George Sanford of the 1st US Cavalry, who served on Pleasonton's staff. Sanford related a conversation that he had with Buford from his sickbed wherein Buford told him this. It can be found in Sanford's published memoir, Fighting Rebels and Redskins. I am completely satisfied that it happened.