Sahih Bukhari Volume 001, Book 003, Hadith Number 127.
Narated By 'Abdullah : While I was going with the Prophet through the ruins of Medina and he was reclining on a date-palm leaf stalk, some Jews passed by. Some of them said to the others: Ask him (the Prophet) about the spirit. Some of them said that they should not ask him that question as he might give a reply which would displease them. But some of them insisted on asking, and so one of them stood up and asked, "O Aba-l-Qasim ! What is the spirit?" The Prophet remained quiet. I thought he was being inspired Divinely. So I stayed till that state of the Prophet (while being inspired) was over. The Prophet then said, "And they ask you (O Muhammad) concerning the spirit... Say: The spirit... its knowledge is with my Lord. And of knowledge you (mankind) have been given only a little)." (17.85)