Musannaf Ibn Abi Shayba 13:3168

Abu Bakr narrated to us, he said: Jarir narrated to us, from Mughira, from al-Sha'bi, who said: Shurayh used to swear an absolute oath regarding a man who claims a debt against his father; if he (the father) swears, then he (the claimant) takes it from him. And if your father has a debt against a person which he claims, then he must establish evidence. If he (the debtor) swears along with your evidence, (he pays); otherwise, he will not give it to you.