Musnad Ahmad 27:549

Abu Mu'awiyah narrated to us, Hajjaj narrated to us, from Atiyyah al-Awfi, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) forbade that fruit be sold until its soundness becomes apparent. He said: They said, "O Messenger of Allah, what is its soundness?" He said: "When its blight departs and its sweetness becomes pure."