Muwatta Malik 1306

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 ‏.‏

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

Reference

Muwatta Malik 1306

In-book

Book 31, Hadith 17