A friend was expounding a midrash to me about Zipporah but was unaware of the provenance. The story involves Zipporah, prior to meeting Moshe, searching for her lost flocks near Har Sinai. She directed prayers to the God of Avraham and then found her sheep drinking water at the foot of the mountain. Noticing that the water seemed to be coming off the mountain itself, she tried to find the source and met an angel. The angel took her by the hand and led her to the summit which was the color of sapphire. The angel said words to the effect of 'My daughter, I've seen your heart and you will become part of a house that will be a blessing, and you will marry its leader. And one day you all will return to this mountain.' She then departed from the mountain, and the spot where she had prayed now had a green bush growing, which later became the burning bush.
Does anyone happen to know a source for this midrash? Many thanks