Sahih al-Bukhari 814
Sahih al-Bukhari · Book 10: Call to Prayers (Adhaan) · To tie the clothes and wrap them properly [in Salat (prayer)]; and whoever gathered his clothes for fear that his private parts may become exposed.
Narrated Sahl bin Sa`d:
حَدَّثَنَا مُحَمَّدُ بْنُ كَثِيرٍ ، قَالَ أَخْبَرَنَا سُفْيَانُ ، عَنْ أَبِي حَازِمٍ ، عَنْ سَهْلِ بْنِ سَعْدٍ ، قَالَ كَانَ النَّاسُ يُصَلُّونَ مَعَ النَّبِيِّ صلى الله عليه وسلم وَهُمْ عَاقِدُو أُزْرِهِمْ مِنَ الصِّغَرِ عَلَى رِقَابِهِمْ فَقِيلَ لِلنِّسَاءِ لاَ تَرْفَعْنَ رُءُوسَكُنَّ حَتَّى يَسْتَوِيَ الرِّجَالُ جُلُوسًا.
The people used to pray with the Prophet (ﷺ) tying their Izars around their necks because of their small sizes and the women were directed that they should not raise their heads from the prostrations till the men had sat straight.