Bulugh al-Maram, Book 6, Hadith 752 (English)

'Uthman bin ’Affan (RAA) narrated that the Messenger of Allah said: “A Muhrim (one in the state of Ihram) must not contract marriage, nor help others contract marriage, nor get engaged to marry." Related by Muslim.

وَعَنْ عُثْمَانَ بْنِ عَفَّانَ ‏- رضى الله عنه ‏- أَنَّ رَسُولَ اَللَّهِ ‏- صلى الله عليه وسلم ‏-قَالَ: { لَا يَنْكِحُ اَلْمُحْرِمُ, وَلَا يُنْكِحُ, وَلَا يَخْطُبُ } رَوَاهُ مُسْلِمٌ 1‏.‏ ‏1 ‏- صحيح.‏ رواه مسلم ( 1409 )‏.‏

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also referenced under: Book 6, Hadith 26 (In-Book)