Musnad Ahmad 30:1750

Surayj narrated to us, Hammad narrated to us, from Abu al-Muhazzam, who said: I heard Abu Hurayrah saying: We were with the Messenger of Allah (may Allah bless him and grant him peace) on a Hajj or an Umrah, and a swarm of locusts met us. We began striking them with our whips and our sticks, and we were killing them. Then it weighed heavily upon us, and we said: "What are we doing while we are in the state of Ihram?" So we asked the Messenger of Allah (may Allah bless him and grant him peace), and he said: "There is no harm."