Musnad Ahmad 171:1

Yazid narrated to us, he said: Ibn Abi Dhi'b informed us, from Sa'id ibn Khalid, from Sa'id ibn al-Musayyib, from 'Abd al-Rahman ibn 'Uthman, who said: A physician mentioned a medicine in the presence of the Messenger of Allah, may Allah bless him and grant him peace, and he mentioned that a frog is put into it. So the Messenger of Allah, may Allah bless him and grant him peace, forbade the killing of the frog.