Muwatta 2122

Malik says: some people say the oath is not admissible with one witness, citing God's word: call to witness two men from among you, and if there are not two men, a man and two women. They say: if he does not produce a man and two women, he has nothing, and does not swear with his witness. Malik said: the proof against those who say this is to ask them: if a man claimed money from another, does the defendant not swear that the right is not upon him? If he swears, the claim falls from him; if he refuses the oath, the claimant swears that his right is true, and his right is established against his partner. This is something on which no one among people disagrees, in any city. ¹

Belongs to the Muwatta, Book of Judgments. Source: the Muwatta. Mentions Malik ibn Anas. Built on in the chapters on testimony (shahada).