Musnad Ahmad 3665
Musnad of Abdullah bin Mas'ood
It was narrated that ‘Abdullah (رضي الله عنه) said: I was walking with the Prophet (ﷺ) in some farmland in Madinah and he was leaning on a palm tree branch. He passed by some of the Jews, who said to one another: Ask him about the spirit (ar-rooh). And some of them said: Do not ask him. So they asked him about the spirit. They said: O Muhammad, what is the spirit? He stood up and leaned on the palm tree branch, and I thought that he was receiving revelation. Then he said: "And they ask you (O Muhammad (ﷺ)) concerning the Rooh (the spirit). Say: “The Rooh (the spirit) 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” (al-Isra' 17:85). And some of thern said: we told you not to ask him.