Muwatta Imam Malik Book of Business Transactions, Hadith 16

Yahya related to me from Malik that he had heard that Umar ibn Abd al-Aziz decided in a case to make a reduction for crop damage.
Malik said, "That is what we do in the situation."
Malik added, "Crop damage is whatever causes loss of a third or more for the purchaser. Anything less is not counted as crop damage."

وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ عُمَرَ بْنَ عَبْدِ الْعَزِيزِ، قَضَى بِوَضْعِ الْجَائِحَةِ . قَالَ مَالِكٌ وَعَلَى ذَلِكَ الأَمْرُ عِنْدَنَا . قَالَ مَالِكٌ وَالْجَائِحَةُ الَّتِي تُوضَعُ عَنِ الْمُشْتَرِي الثُّلُثُ فَصَاعِدًا وَلاَ يَكُونُ مَا دُونَ ذَلِكَ جَائِحَةً .