It is narrated from Ibn Umar (may Allah be pleased with them both) that he said: "Whoever swears an oath and then affirms it, then (upon breaking it) it is obligatory for him to free a slave."
Hadith Referenceمسند الإمام الشافعي / كتاب القضاء والأحكام والدعاوى والبينات واليمين ومع الشاهد والأيمان والشهادات / 1725
Hadith Takhrijصحيح اخرجه البيهقي : 56/10 - وفى المعرفة السنن والآثار له (5814)۔