Hawzah News Agency – Imam al-Sadiq peace be upon him says:

إنّ المؤمنَ أشدُّ مِن زُبَرِ الحديدِ، إنّ زُبرَ الحديدِ إذا دَخلَ النّار تَغيّرَ، وإنّ المؤمنَ لو قُتِلَ ثُمّ نُشِرَ ثُمّ قُتِلَ لم يَتغيّرْ قلبُهُ.

'Verily the believer is stronger than pieces of iron, for when a piece of iron enters the fire it changes, whereas the believer, were he to be killed then resurrected and then killed again, his heart would never change.'

Source: Bihar al-Anwar, v. 67, p. 303, no. 34