To the op:
http://www.sermonaudio.com/sermoninfo.asp?SID=6190764259
Start at the 20 minute mark. Or page 7 of the pdf:
http://media.sermonaudio.com/mediapdf/6190764259.pdf
The R.C. Sproul teaching he quotes comes from Sproul's book "The Holiness of God."
The Washer quote is as follows:
"I have always asked myself: Why on earth did they cover their feet? I mean, what does feet have to do with holiness? What does it mean? I never could figure that out honestly until one day I was listening to R C Sproul. And I think he hit upon the thing. He said, “Feet are always associated with clay and clay with the creature, the dust of the earth. And these majestic beings, even in the presence of God were bowing their heads and covering their feet acknowledging that they were a creature and they were not God. And the place that they held was a place not of rights. It was not their right to be there, but their privilege, graciously given to them by God himself.”