With that exhortation near the end of a prepared speech, Mahalia Jackson inspires one of the great extemporaneous speeches in history, according to Clarence Jones:

He moves the text of the speech to the left side of the lectern, grabs the lectern, looks out on those more than 250,000 people assembled and thereafter begins to speak completely spontaneously and extemporaneously.

Watch the whole speech here.

An interesting side note: Jones was MLK’s personal lawyer and hand wrote the copyright symbol on every copy of the speech. That tiny decision may be worth millions to MLK’s estate today.