Ubaidullah bin al-Akhns عبيد الله بن الأخنس النخعي أبو مالك · The Book of Oaths and Vows

"The Messenger of Allah said: 'Whoever says: I have nothing to do with Islam, if he is lying then he is as he said, and if he is telling the truth, his Islam will not be sound.'"

"The Messenger of Allah said: 'If you swear an oath, offer expiation for your oath, then do that which is better.'"