Al-Lu'lu Wal-Marjan 1780 — The Book About the Characteristics of the Hypocrites and Orders Concerning Them.

'Abdullah bin Mas'ud Narrated from:

While I was walking along with the Prophet صلى الله عليه وسلم through the ruins of Al-Madîna and he was reclining on a date-palm leafstalk, some Jews passed by. Some of them said to the others: Ask him (the Prophet صلى الله عليه وسلم ) about the Rûh (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 Abul-Qâsim! What is the Rûh (the spirit).”The Prophet صلى الله عليه وسلم remained quiet. I thought that he was being inspired divinely. So I stayed till that state of the Prophet صلى الله عليه وسلم was over. The Prophet صلى الله عليه وسلم then said,“And they ask you (O Muhammad صلى الله عليه وسلم ) concerning the Ruh (the spirit) — Say: The Ruh (the spirit) – it is one of the things the knowledge of which is only with my Lord. And of knowledge you (mankind) have been given only a little.” (V.17:85)( Sahîh Al-Bukhârî, Hadîth No. 127, Vol. 1 )