صحيح البخاري
Sahih Bukhari
كتاب الأدب
The Book of Al-Adab (Good Manners)
باب من لم ير إكفار من قال ذلك متأولا أو جاهلا:
(74) Chapter. Whoever does not consider as Kafir the person who says that (i.e., O Kafir), if he thinks that what he says is true, or if he is ignorant of the seriousness of such saying.