Musannaf Ibn Abi Shayba 33:259

Mu'awiyah ibn Hisham narrated to us, from Hisham ibn Sa'd, from Zayd ibn Aslam, from his father, who said: I was walking with Umar ibn al-Khattab when he saw a date lying on the ground. He said, "Pick it up." I said, "And what shall I do with one date?" He said, "A date and a date, until they accumulate." So I picked it up. Then he passed by a place where dates were being dried, and he said, "Throw it there."