It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) concerning the verse "except when you guard yourselves against them as a precaution (tuqah)" [Surah Al Imran: 28] ; he said: "Tuqah" (taqiyyah) means only saying (such a thing) with the tongue while the heart is fully at rest in faith. But he must not (even under compulsion) stretch out his hand to kill, nor go towards any sin, for in that no excuse is accepted. This hadith has a sahih (authentic) chain of narration, and the two Shaykhs (Bukhari and Muslim) did not record it.