Before ye all run out to get a cup of spring elixer, stop for a minute and think about what you are doing. Sap is the blood of the tree, rising now in the wonderfully exultant first heartbeat of the forest. And we wound the tree to colloct this sap.
We have to be very careful to staunch the wound when we are finished, so that the tree does not bleed to death. For that reason, it would not make sense to use a piece of the tree we are wounding to make the plug, would it? It is much better to bring a clean piece of dry wood with us, or look for a clean piece of broken wood nearby, possibly a branch from winter's storm damage.
And clean is very important, not just for the plug, but for our tools. We don't want to inject an infection while we are at it. For this reason, I always wipe any blade I am about to use on living wood with teatree oil and clovebud oil.