Ан-Нур 24:25

يَوْمَ تَشْهَدُ عَلَيْهِمْ أَلْسِنَتُهُمْ وَأَيْدِيهِمْ وَأَرْ‌جُلُهُم بِمَا كَانُوا يَعْمَلُونَ ﴿24﴾

On the Day their tongues, their hands and their legs will testify against them of what they used to do. - 24:24.

The Day on which their own tongues, hands and feet will speak against them and will testify against their crimes, they will be punished precisely according to the crimes they had committed. It is related in Ahadith that on the Dooms Day those who will admit their sins will be pardoned by Allah Ta’ ala, and He will conceal their sins from the crowd of people. If anyone will deny his sins and will claim that he did not commit them, and say that the angels did not write his record of actions correctly, then his speech will be taken away from him and his hands and feet will be asked to testify, who will speak and testify. Another verse الْيَوْمَ نَخْتِمُ عَلَىٰ أَفْوَاهِهِمْ (Today We set a seal on their mouths, - 36:65) states this very point. This verse says that their mouths will be sealed, but the verse under discussion (of Surah An-Nur) says that their tongues will testify. There is no contradiction in the two verses in that they will not be able to use their tongues at their discretion. In other words they will have no control on tongues to say anything wrong, as they have in this world. Instead their tongues will say only the right things against their intent and wish. It is also possible that at a certain time their mouths and tongues will be completely sealed and then the tongues are commanded to speak the truth (Only Allah knows best).