Source is the semantic role of the following referents:
John fell off the chair.(with a motion verb)
The baby crawled from the kitchen to the door. (with a locomotion verb)
John threw the knife into the box. (with a propulsion verb)
NOTE: With propulsion verbs, the source is coreferential with the agent.
As the entity from which a physical sensation emanates:
John smelled the odor of onions. (with a sensation verb)
The people watched the performance of the dancers. (with an attention verb)
The mother told her child a story. (with a speech verb)
As the original owner in a transfer:
John obtained an application form from the office. (with an acquisition verb)
John bought the book from Tom. (with a transfer verb)
John grabbed the book from Tom. (with a grab verb)
Longacre, Robert E. 1983.The grammar of discourse. New York: Plenum.