Fourth condition: the garment must be loose-fitting, not tight so that it would outline any part of her body.
The very purpose of clothing is to remove temptation, and this is achieved only with a wide, flowing garment. A tight outfit, even if it hides the color of the skin, still reveals the size of her body—or parts of it—and presents that image to men’s eyes, an obvious cause of corruption and its invitation. Hence the garment is required to be ample.
Usāmah ibn Zayd said:
“The Messenger of Allah ﷺ clothed me with a thick qubṭiyyah (an Egyptian linen robe) that Dihyah al-Kalbī had given him. I let my wife wear it. He asked, ‘Why did you not wear the qubṭiyyah yourself?’ I replied, ‘I put it on my wife.’ He said, ‘Instruct her to place a ghilālah (slip garment) beneath it, for I fear it may describe the shape of her bones.’” 1
Thus the Prophet ﷺ commanded that a woman wear a ghilālah—an undergarment worn beneath the outer clothes—under the qubṭiyyah to prevent her figure from being outlined. As established in legal theory, such a command denotes obligation