Maliks Muwatta Book 31, Hadith Number 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.”