Musnad Ahmad 26:698

Waki’ narrated to us, al-A’mash narrated to us, from Ibrahim, from ‘Alqamah, from ‘Abdullah, who said: I was walking with the Prophet, peace and blessings of Allah be upon him, in a cultivated field in Medina. He passed by a group of Jews, and some of them said to others: "Ask him about the soul." Others said: "Do not ask him." Then they said: "O Muhammad, what is the soul?" He said: He stood up, leaning on a palm branch, and I was behind him. I thought that revelation was coming to him. Then he said: "{They ask you about the soul. Say, 'The soul is of the affair of my Lord, and you have not been given of knowledge except a little'}" [Al-Isra: 85]. He said: Some of them then said: "We already said: Do not ask him."