“Words are mere man-given symbols for thoughts and feelings, and they are grossly insufficient to express the thoughts in my mind and the feelings in my heart that I have for you,” Hoover wrote to Tolson in 1943. “I hope I will always have you beside me.”
Apparently that quote comes from
this article which says that the Hoover letter to Tolson was found in Tolson's FBI personnel file. The FBI FOIA website has all their material on Tolson, apparently including the personnel file,
here. This material is too voluminous for me to go through right now, but it would be interesting to see a copy of the original letter from Hoover, with his signature.