From Sahih Muslim (Hadith 5846):
Abu Huraira reported Allah's Messenger (ﷺ) 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
Reference: Muslim 5846