Muwatta Malik, Book 9, Hadith 53 (USC-MSA) ⓘ
Yahya related to me from Malik from Zayd ibn Aslam that Umar ibn al-Khattab said, "You should not pray while you are holding your bowels."
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ زَيْدِ بْنِ أَسْلَمَ، أَنَّ عُمَرَ بْنَ الْخَطَّابِ، قَالَ لاَ يُصَلِّيَنَّ أَحَدُكُمْ وَهُوَ ضَامٌّ بَيْنَ وَرِكَيْهِ .
also referenced under: Book 9, Hadith 384 (In-Book)