Musnad Abu Ya'la al-Mawsili 117:12

Hadaba ibn Khalid narrated to us, Ubayd ibn Muslim, the owner of the Sabiri, narrated to us, from Thabit, from Anas: That the Messenger of Allah (peace and blessings of Allah be upon him) said: "The likeness of the believer is like the stalk of grain; it leans sometimes and stands upright sometimes."