A little boy knocked on the warden's door at Sing Sing, then in tears
Said "Sir, I understand that 83968 lives here
Well, he's my daddy, sir, and I just got to be near him
And if you can't let him out then you just got to let me in"
Let me in, let me in, 83968's my dad
I watched them bury mama
Now he's all in the world I have
I can break your little rocks with a hammer in my hand
I just got to be near daddy, Mr Warden please let me in
Well I checked through California, Alcatraz out on the rock
From Folsom on to Leavenworth, no one had my pop
When I found this newspaper clipping in my mama's souvenirs
And now I'm here in Sing Sing, 83968 lives here
The Grateful Dead played this on at least a couple of occasions in 1969 and 1970. It also features on a tape known as "The Garage Jam" from 1970, supposedly with Garcia, Weir and Pete Sears, and as a studio rehearsal on 21 November 1970 by Garcia, Weir and Duane Allman.
These are the lyrics from those versions(thanks to Matt Schofield).