In the seventh heaven I saw rivers of light, such that the light that was coming from them made the eyes blind. Rivers of Darkness were also there that were covered over with ice and the sound of thunder crashing could be heard. I was busy looking at these rivers when Jibra'il said to me, "O' Muhammad ‑ be thankful to Allah for the graces and bounties that have been chosen for you."
I prayed, "O' Allah by the truth of Your Power and Glory, keep my faith firm." Then I said to Jibra'il, "This is a beautiful and amazing scene." He replied, "This is just a portion of the creations of your Lord ‑ the Creator who has created everything ‑ some of which you have seen and some of which you have not even yet seen.
Jibra'il continued by saying, "Between Allah and His creations, there exists 90,000 layers of concealment and the closest of creations to Allah are Isra'fil and myself; and between Allah and us exist four veils: Light, Darkness, Cloud and Water."
From Bihar al-Anwar, Vol. 18, p. 326-327, Bab 3 - Ithbaat al-Mi`raaj wa ma`anaahu wa kayfiyyatuhu wa sifatuhu wa ma jaraa feehi wa wasf al-buraaq, Hadith 34.