Musannaf Ibn Abi Shayba 7:198

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Ayyub, from Nafi’, who said: Ibn ‘Umar used to, when he swore an oath, feed [a poor person] a mudd. And if it was more emphatic (awkad), he would free a slave. I said to Nafi’: "What is the emphatic [oath]?" He said: "To repeat the oath regarding the same single matter."