Musnad Ahmed: Hadith 3875

Whilst the Prophet (ﷺ) was in some farmland, leaning on a palm tree branch, some of the Jews came to him and asked him about the spirit. He remained silent, then he recited this verse to them: "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).