Musannaf Ibn Abi Shayba 16:310

Abu Bakr narrated to us, he said: Waki’ narrated to us, from al-A’mash, from Mundhir, from al-Rabi’ ibn Khuthaym: That he said to his family, "Make me some khabis (a sweet dish)." He said: So they made it. Then he invited a man who had a mental affliction. He said: Al-Rabi’ began to feed him by hand while his saliva was flowing. When he had eaten and left, his family said to him, "We went to the trouble of making it for him, and you fed it to someone who does not even know what he has eaten." Al-Rabi’ said, "But Allah knows."