Sahih Muslim
The Book on Salutations and Greetings (Kitab As-Salam)
Book Number: 026 / Hadith Number: 5565
This hadith has been reported on the authority of Abu Huraira through another chain of transmitters (and the words are): - He who killed a gecko with the first stroke for him are ordained one hundred virtues, and with the second one less than that and with the third one less than that.