Short answer
Killing the gecko (wazagh) is recommended in the sunnah: the Prophet ﷺ ordered it and promised extra reward for killing one with the first blow. This applies to the house gecko, not to every lizard. Other harmless lizards should be left alone, and no lizard should be tortured or burned.
Killing the wazagh (house gecko) is mustahabb (recommended). Other lizards take the general rule: they may be killed if harmful and should be left if harmless.
The four schools agree on this.
Explanation
The Prophet ﷺ ordered that the wazagh be killed, and said it used to blow on the fire in which Ibrahim was thrown. He also said that whoever kills a wazagh with the first blow gets a certain number of good deeds, and fewer with the second blow. Imam al-Nawawi titled the chapter in his commentary on Sahih Muslim 'the recommendation to kill the wazagh'.
Who is the wazagh? The scholars of language and hadith describe it as the small lizard that lives in houses and on walls, which today we call the house gecko. It is not a name for every reptile. Large monitor lizards, garden lizards and pet lizards like bearded dragons are not covered by this hadith. The dabb, a desert lizard, was even eaten in front of the Prophet ﷺ, though the Hanafi school does not allow eating it.
Points to keep in mind:
- The reward for a single blow encourages a quick kill. Do not chase it to torment it, and do not burn it.
- Killing it is recommended, not obligatory. If you cannot, there is no sin.
- The story about Ibrahim's fire explains the hadith. It does not mean today's geckos carry any guilt; it is Allah's choice what He commands.
- Do not go looking for lizards in the wild to kill them; the hadith is about the ones near people.
The four schools agree that killing the gecko is recommended. For other creatures, see killing snakes and killing insects.
The evidence
Sahih al-Bukhari 3359
Sahih... أمر بقتل الوزغ ...
"He ordered that the gecko be killed."
The basis for the recommendation.
Sahih Muslim 2240
Sahih... من قتل وزغة في أول ضربة فله كذا وكذا حسنة ...
"Whoever kills a gecko with the first blow will have such and such good deeds."
Extra reward for a quick kill.
Where the ruling is recorded
- Sharh Sahih Muslim (al-Nawawi), Kitab al-Salam, chapter on the recommendation to kill the gecko. Explains that killing the gecko is recommended and the reward is greater for the first blow.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.