Aishah (may Allah be pleased with her) says that Abu Bakr al-Siddiq (may Allah be pleased with him) used to support Mistah bin Uthathah. When he said what he said about Aishah (may Allah be pleased with her), Abu Bakr swore that he would not benefit him. When Allah revealed: "Let not those of virtue and means among you swear against giving to near relatives, the poor and the emigrants for the sake of Allah" [Surah al-Nur: 22], Abu Bakr al-Siddiq (may Allah be pleased with him) said: "I love that Allah should forgive me," and he resumed Mistah's allowance and gave the expiation for his oath.
Imam al-Shafi'i (may Allah have mercy on him) said: The saying of Allah, "And indeed they utter an evil word and a falsehood" [Surah al-Mujadilah: 2], then Allah placed expiation in it.
The Shaykh (may Allah have mercy on him) said: Expiation is obligatory in it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19875