Muwatta Imam Malik (Riwayat Yahya)

Hazrat Jameel bin Abdur Rahman used to come to Hazrat Umar bin Abdul Aziz when he would judge among the people. If someone made a claim against another, and if it was apparent that there was association, connection, or relationship between the claimant and the defendant, then he would take an oath from the defendant; otherwise, he would not take an oath.

Imam Malik, may Allah have mercy on him, said: In our view, this is the ruling: if a person makes a claim against another, it will be examined. If the claimant is known to have association and connection with the defendant, then an oath will be taken from the defendant. If he swears an oath, the claim of the claimant will be invalidated. If he refuses, then an oath will be taken from the claimant. If he swears an oath, he will receive his right.