But if he has another garment on, then there is nothing disliked about it, for this is how the person in the state of iḥrām dresses (1), and the Prophet ﷺ did so (2).
Iḍṭibāʿ means uncovering one’s right shoulder and placing both ends of the upper wrap over the left shoulder.
The reason it is disliked here is that the garment may fall, exposing the private parts. If there is an actual fear that the private parts will be exposed, then it is prohibited.
It has also been said that it means placing the upper wrap over one’s head and then letting its two ends hang down to one’s feet (3). Thus, there are three descriptions of wrapping oneself in al-samma’. If you consider each of these descriptions, you will find that it conflicts with Allah’s saying: “O children of Adam, take your adornment at every place of prayer.” [al-A‘raf: 31] Wearing one’s adornment in this manner entails a degree of deficiency, for wearing one’s adornment properly means wearing it in the manner people are accustomed to wear it, such that it covers adequately and is
(1) Ahmad (2:33), Ibn Khuzaymah, no. 2601, Ibn al-Jarud, no. 416, and others narrated from Abd al-Razzaq, from Mamar, from al-Zuhri, from Salim, from Ibn Umar, attributing it to the Prophet: “... Let one of you enter ihram wearing an izar (lower garment), a rida (upper wrap), and sandals ...” Ibn al-Mundhir said: “This is established from the Messenger of Allah.” Al-Mughni, 5:76.
I say: Its chain of transmission is sahih (authentic). Its basis is found in the two Sahihs through the narration of Malik, from Nafi, from Ibn Umar, with the same wording. Abd al-Razzaq, from Mamar, from al-Zuhri, from Salim, added this wording. Ibn Hajar said: “It is a good addition.” Al-Fath, commentary on hadith no. 1542.
(2) Muslim narrated in The Book of Prayer, chapter: Placing the Right Hand upon the Left, no. 401, from Wail ibn Hujr, who saw the Prophet raise his hands when he began the prayer and said the takbir; he then wrapped himself in his garment and placed his right hand over his left. When he wished to bow, he brought his hands out from the garment, then raised them both ... the hadith.
(3) See: Al-Insaf, 3:249-250.