It is recorded, on the "Woodstock" record, which is six sides long and has selected songs and stage announcements from the entire festival. This is exactly what was said- I'm transcribing from my record now:

Uh, to get back to the, uh, the warning that I've received you may take it with how many however many grains of salt you wish, that the brown acid that is circulating around us is not specifically too good. Uh, it's suggested that you do stay away from that, course it's your own trip, so be my guest, but, uh, please be advised that there is a warning on that one ok?