Musannaf Ibn Abi Shayba 33:260

'Abdah ibn Sulayman narrated to us, from Yahya ibn Sa'id, from 'Abdullah ibn 'Amir, who said: I went out with 'Umar, and I did not see him pitch a tent until he returned. He said: I said, "With what would he seek shade?" He said: "He would throw a leather mat over a tree and seek shade under it."