Musnad Ahmad 27:1072

'Abd al-Razzaq narrated to us, Sufyan informed us, from Ibn Abi Layla, from al-'Awfi, from Ibn 'Umar, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Do not trade fruits until their soundness becomes apparent." He said: And what is the appearance of their soundness? He said: "Their blight departs and their goodness becomes pure."