My father further explained in his letter: "Prostrate on the earth or on what grows from the earth. Do not prostrate on the ḥuṣur (reed mats) of Medina because their bindings are made of leather. Do not prostrate on hair, wool, leather, silk, glass, iron, brass, bronze, lead, copper, feathers, or ashes. If the earth is hot and you fear burning your forehead, or if it is a dark night and you fear scorpions or thorns that might harm you, there is no issue with prostrating on your sleeve if it is made of cotton or linen. If there is a swelling on your forehead, dig a small hole so that when you prostrate, the swelling can fit inside. If you have an ailment on your forehead that prevents you from prostrating, prostrate on the right side of your forehead. If that is not possible, then prostrate on the left side of your forehead. If you cannot do that, prostrate on the back of your hand. If even that is not possible, then prostrate on your chin, for Allah
سبحانه وتعالى, the Mighty and Majestic, says: 'Those who were given knowledge before it, when it is recited to them, fall down prostrating on their chins.. and it increases them in humility' (Surah Al-Isra,
17:107-109). There is no issue with standing and placing your palms, knees, and big toes on surfaces other than earth. You should bring your nose close to the ground, and for placing the forehead, the amount from the hairline to the eyebrows equivalent to a dirham suffices. Your prostration should be like a thin, gaunt camel lowering itself, such that no part of your body rests on another."
Commentary #2