From the YouTube page:
At the March on Washington in 1963 Mahalia sang in front of 250,000 people "I've Been 'Buked, and I've Been Scorned", in which Martin Luther King, Jr. made his famous "I Have a Dream" speech. She also sang "Take My Hand, Precious Lord" at the funeral of her friend Martin Luther King, Jr. She was one of his favorite gospel singers. Indeed, her good friend Martin Luther King said "a voice like hers comes along once in a millennium". In addition to sharing her singing talent with the world, she mentored the extraordinarily gifted Aretha Franklin; she was a close friend of Aretha's father, C. L. Franklin, and a frequent guest in the Franklin home.