Sahih Muslim
The Book on Salutations and Greetings (Kitab As-Salam)
Book Number: 026 / Hadith Number: 5564
Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: He who killed a gecko with the first stroke for him is such and such a reward, and he who killed it with a second stroke for him is such and such reward less than the first one, and he who killed it with the third stroke for him is such and such a reward less than the second one.