From Sahih Bukhari (Hadith 7462):
Narrated Ibn Mas`ud:While I was walking in company with the Prophet (ﷺ) in one of the fields of Medina, the Prophet (ﷺ) was reclining on a palm leave stalk which he carried with him. We passed by a group of Jews. Some of them said to the others, "Ask him about the spirit." The others said, "Do not ask him, lest he would say something that you hate." Some of them said, "We will ask him." So a man from among them stood up and said, 'O Abal-Qasim! What is the spirit?" The Prophet (ﷺ) kept quiet and I knew that he was being divinely inspired. Then he said: "They ask you concerning the Spirit, Say: The Spirit; its knowledge is with my Lord. And of knowledge you (mankind) have been given only a little
Reference: Bukhari 7462