Musnad Abu Ya'la al-Mawsili 153:41

And from Ibn Shihab, from Salim, from Abdullah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not trade fruit for fruit, and do not trade fruit until its soundness becomes apparent." He said: And the Messenger of Allah (peace and blessings of Allah be upon him) did not appoint a judge, nor did Abu Bakr, nor Umar, until it was in the latter part of his (Umar's) time. He said to Yazid ibn Ukht Namir: "Take care of some of the matters for me," meaning the minor ones.