Nafi' narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that he used to give the expiation for an oath by feeding ten poor persons, one mudd of wheat for each person, and sometimes he would also free a slave when the oath was emphatic.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19973