Hadith 33: Burden of Proof
Ibn Abbas reported: The Messenger of Allah, peace and blessings be upon him, said:
لَوْ يُعْطَى النَّاسُ بِدَعْوَاهُمْ لَادَّعَى رِجَالٌ أَمْوَالَ قَوْمٍ وَدِمَاءَهُمْ وَلَكِنَّ الْبَيِّنَةَ عَلَى الْمُدَّعِي وَالْيَمِينَ عَلَى مَنْ أَنْكَرَ
If people were given in accordance with their claims, then men would claim the wealth and lives of other people. Rather, the burden of proof is on the claimant and an oath is an obligation upon him who denies the claim.
Source: Sunan Al-Kubra