Ash-Shama'il Al-Muhammadiyah, Book 56, Hadith 414 (In-Book) ⓘ
'Abdullah bin Mubarak said: "If one becomes a judge or arbitrator, then they should always follow that which has been narrated."
حدثنا محمد بن علي، قال سمعت أبي يقول: قال عبد الله ابن المبارك: إذا ابتليت بالقضاء فعليك بالأثر.
also referenced under: Book 55, Hadith 396 (English)