Musnad Ahmad 31:649

Yazid narrated to us, Sa'id informed us, from Qatadah, from Sa'id ibn al-Musayyib, that Abu Sa'id al-Khudri narrated to them that a servant of the Prophet, peace and blessings of Allah be upon him, came to him one day with fresh, moist dates. The dates of the Prophet, peace and blessings of Allah be upon him, were *ba'l* (dates grown with rainwater) and contained dryness. The Prophet, peace and blessings of Allah be upon him, said: "From where did you get these dates?" He said: "This is a *sa'* (measure) that we bought for two *sa's* of our dates." The Prophet, peace and blessings of Allah be upon him, said: "Do not do that, for this is not permissible. Rather, sell your dates, and buy whichever dates you wish."