Shaykh bin Baz was asked about writing in the Mushaf, he replied:
It is impermissible to write anything, nothing is to be written in the Mushaf. It should be devoid of anything but the speech of Allah – glorified and exalted be He – no footnotes should be written, neither any symbols of tajweed nor anything else. Because it’s a must for the Mushaf to be safeguarded and solely contain the speech of Allah – glorified and exalted be He.
Translated by Abu Arwa Ali bin Mushtaq Mir