Musnad Abu Ya'la al-Mawsili 115:239

Muhammad ibn Yahya narrated to us, Fahd ibn Hayyan narrated to us, Hammam narrated to us, from Qatadah, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The likeness of the believer is like the stalk of grain; it leans at times and stands upright at times."