“When az-Zubayr married me, he had neither land, nor wealth, nor slave, nor anything else like it, except a camel to get water and his horse. I used to graze his horse, provide fodder for it, look after it and ground dates for his camel. Besides this, I grazed the camel, made arrangements for providing it with water and patching up his leather bucket and kneading the flour. I was not very good at baking the bread, so my female neighbors used to bake bread for me and they were sincere women. And I used to carry on my head, the date-stones from the land of az-Zubayr which the Prophet sallallahu ‘alayhi wa sallam had endowed him and it was a distance of two miles from Madinah. One day, as I was carrying the date-stones upon my head, I happened to meet Allah’s Messenger sallallahu ‘alayhi wa sallam, along with a group of his Companions. He called me and told the camel to sit down so that he could make me ride behind him. I felt shy to go with men and I remembered az-Zubayr and his ghirah (ghirah is the sense of pride that a man has which causes him to dislike his wives, daughters or sisters from being seen or heard by strangers – it is this ghirah which makes a man protective about his women) and he was a man having the most ghirah. The Messenger sallallahu ‘alayhi wa sallam understood my shyness and left. I came to az-Zubayr and said, ‘The Messenger of Allah met me as I was carrying date-stones upon my head and there was with him a group of his Companions. He told the camel to kneel so that I could mount it but I felt shy from him and I remembered your ghirah.’ Upon this az-Zubayr said, ‘By Allah, the thought of you carrying date-stones upon your head is more severe a burden to me than you riding with him.’ I led this life of hardship until Abu Bakr sent me a female servant who took upon herself the responsibility of looking after the horse and I felt as if she had emancipated me.”
[Reported in Sahih al-Bukhari (eng. trans.) vol. 7, p. 111, no. 151]
Imam an-Nawawi said in his commentary (on sahih Muslim) of the hadith (my own translation take it with the necessary care:
وفيه جواز إرداف المرأة التي ليست محرما إذا وجدت في طريق قد أعيت ، لا سيما مع جماعة رجال صالحين ، ولا شك في جواز مثل هذا
From this we can conclude that it is permissible to seat a non-mahram woman (behind one on a mount) if she is exhausted on the road, especially in a company of good or upright people, and there’s no doubt that such things are permissible!
Ibn hajjar al-‘Asqalani concluded with a more general statement in his fath al-Bari (again my own translation):
وفيه جواز ارتداف المرأة خلف الرجل في موكب الرجال
From this we can conclude that it is permissible to seat (on a mount) a woman in a group of men
He also pointed to the fact that Asma’ didn’t hide herself or use a hijab or something similar, before commenting that this might have happened before the order for hijab!
Imam an-Nawawi also quoted a rather opposite statement of qadi ‘Iyad saying:
وقال القاضي عياض : هذا خاص للنبي صلى الله عليه وسلم بخلاف غيره ، فقد أمرنا بالمباعدة من أنفاس الرجال والنساء ، وكانت عادته صلى الله عليه وسلم مباعدتهن لتقتدي به أمته ، قال : وإنما كانت هذه خصوصية له لكونها بنت أبي بكر ، وأخت عائشة ، وامرأة الزبير ، فكانت كإحدى أهله ونسائه ، مع ما خص به صلى الله عليه وسلم أنه أملك لإربه . وأما إرداف المحارم فجائز بلا خلاف بكل حال
And al-Qadi ‘iyad said: this is special for the Prophet (Allahs prayers and blessings be upon him) in contrary to others, as he asked us to create a distinction between men and women, and it was his custom to do so, so that his Ummah may follow him. He further said: and the special for him () here was that she was the daughter of abu Bakr, the sister of ‘Aishah and the wife of az-Zubair, so she was as one of his family or wives, plus the special about him () that he had most control of his desires (see for example sunan abi Dawod). When it comes to seat a mahram this is permissible without any opposite view in any case.