Maliks Muwatta Book 20, Hadith Number 245.
Section : The Fidya for Killing Locusts in Ihram.
Yahya related to me from Malik from Yahya ibn Said that a man came to Umar ibn al-Khattab and asked him about some locusts he had killed while he was in ihram. Umar said to Kab, “Come, let’s decide.” Kab said, “A dirham,” and Umar said to Kab, “You can find dirhams. A date is better than a locust.”