I believe what you're referring to is the inscription on Susy's headstone:

                Warm summer sun
                        Shine kindly here
                Warm southern wind
                        blow gently here
                Green sod above
                        Lie light, lie light
                Good night, dear heart,
                        Good night, good night.

        This was written, not by Twain, but by an Australian poet, Robert
Richardson. This is described, with a photo of the stone, in Mark Twain in
Elmira, edited by Herbert Wisbey and Bob Jerome, published by the Mark
Twain Society.

        Mark Woodhouse
        Mark Twain Archives
        Elmira College