Does anybody recognize this from Douglas Adams' writings?

The closest in Hitchhikers is in Life, the Universe, and Everything when Prak (a minor character) was given too much truth serum and proceeded to tell the WHOLE truth.

However, Prak did not distill all of the world’s religions into a two-word phrase. Instead, he told Arthur Dent where he could find God’s final message; ”‘We apologize for the inconvenience.”.

Try this as a pointer: link

You seem to be mixing two different sections of H2G2 the being nice bit comes from the first book as @elbanditoroso says where as the truth serum is like @jerv said from Life the Universe and Everything

Here is a link to the script from the radio adaptation of Life the Universe and Everything with Prak’s bit.

You’ll never believe some of the things about frogs HAHAH

….and now I need to go reread the whole series again. I’ve been thinking about reading through them again lately, but this just confirms to me that I should. :)

I think you might be conflating two passages: the one @jerv linked and this one:

“And then, one Thursday, nearly two thousand years after one man had been nailed to a tree for saying how great it would be to be nice to people for a change, a girl sitting on her own in a small café in Rickmansworth suddenly realized what it was that had been going wrong all this time, and she finally knew how the world could be made a good and happy place. This time it was right, it would work, and no one would have to get nailed to anything.”