Muwatta Malik · Hadith 381

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ زَيْدِ بْنِ أَسْلَمَ، أَنَّ عُمَرَ بْنَ الْخَطَّابِ، قَالَ لاَ يُصَلِّيَنَّ أَحَدُكُمْ وَهُوَ ضَامٌّ بَيْنَ وَرِكَيْهِ ‏.‏

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

Gradings are classical jarḥ-wa-taʿdīl, name-matched to a rijāl index (not individually re-verified).