Musnad Ahmad 26:141

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, and he was leaning upon a palm branch. He passed by a group of Jews, and some of them said to others: 'Ask him about the soul.' Some of them said: 'Do not ask him.' Then they asked him about the soul, and they said: 'O Muhammad, what is the soul?' He stood up and leaned upon the palm branch. I thought that revelation was coming to him. Then he said: 'And they ask you, [O Muhammad], about the soul. Say, "The soul is of the affair of my Lord. And mankind have not been given of knowledge except a little."' [Al-Isra: 85]. He said: Then some of them said: 'We told you not to ask him.'"