"The number of dust on the earth could be counted..."

In the video documentary Go Beyond Samsara - Part 2 , it mentions that Buddha had said: > "The number of dust on the earth could be counted, but the number of > beings that had been one's parents is in no case calculable" I'd like to see this in its fuller context. Where in the pali canon or other scriptures did the buddha say this and can you point me to it? I have researched and I can only find similar biblical references like here . Note: The video link above will play at the very moment in question.