Abu Abd al-Rahman al-Sulami (may Allah have mercy on him) said: Dawud (peace be upon him) was commanded to judge, and he became anxious. Allah Almighty revealed to him: "Ask them for an oath by My name and for proofs." He said: That is the "decisive speech" [Surah Sad: 20].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20722