From Sahih Muslim (Hadith 2883):
Abdullah b. Ma'qil said:I sat with Ka'b (Allah be pleased with him) and he was in the mosque. I asked him about this verse:" Compensation in (the form of) fasting, or Sadaqa or sacrifice." Ka'b (Allah be pleased with him) said: It was reveal- ed In my case. There was some trouble in my head. I was taken to the Messenger of Allah (ﷺ) and lice were creeping upon my face. Thereupon he said: I did not think that your trouble had become so unbearable as I see. Would you be able to afford (the sacrificing) of a goat? I (Ka'b) said: Then this verse was revealed:" Com- pensation (in the form of) fasting or alms or a sacrifice." He (the Holy Prophet) said: (It Implies) fasting for three days, or feeding six needy perscins, half sa' of food for every needy person. This verse was revealed particularly for me and (now) Its applica- tion is general for all of you
Reference: Muslim 2883