كتاب النكاح
16
The Book of Marriage
(5)
Chapter: The prohibition of marriage for one who is in Ihram, and it is disliked for him to propose marriage
(5)
باب تَحْرِيمِ نِكَاحِ الْمُحْرِمِ وَكَرَاهَةِ خِطْبَتِهِ ‏‏

Nubaih b. Wahb reported that 'Umar b. Ubaidullah intended to marry Talha b. 'Umar with the daughter of Shaiba b. Jubair; so he sent a messenger to Aban b. Uthman to attend the marriage, and he was at that time the Amir of Hajj. Aban said:

I heard 'Uthman b. 'Affan say that Allah's Messenger (ﷺ) had stated: A Muhrim must neither marry himself, nor arrange the marriage of another one, nor should he make the proposal of marriage.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ نَافِعٍ، عَنْ نُبَيْهِ بْنِ وَهْبٍ، أَنَّفَقَالَ أَبَانٌ سَمِعْتُ عُثْمَانَ بْنَ عَفَّانَ يَقُولُ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ لاَ يَنْكِحُ الْمُحْرِمُ وَلاَ يُنْكَحُ وَلاَ يَخْطُبُ ‏"‏ ‏.‏
Reference : Sahih Muslim 1409 a
In-book reference : Book 16, Hadith 49
USC-MSA web (English) reference : Book 8, Hadith 3278
  (deprecated numbering scheme)