Len recorded his song on his Columbia album 'Lovin' People' (1968, I think). He did it on Pete Seeger's "Rainbow Quest" TV series. You can see a much younger Len doing the song for a much younger Pete on (where else ?) YouTube here:

http://www.youtube.com/watch?v=0qH5_0Etg_I

It's a long time ago, but I think Len's song reminded MLK of the phrase. May have been the other way 'round.

By me, it's just an old, old southern saying, black and white.