Musnad Ahmad 30:1538

Hassan narrated to us, Zuhair narrated to us, from Suhayl ibn Abi Salih, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever kills a gecko with the first blow, for him is such and such a number of good deeds. And whoever kills it with the second, for him is such and such a number of good deeds. And whoever kills it with the third, for him is such and such a number of good deeds." Suhayl said: The first is more.