Arabic text
عن عائشة رضي الله عنها قالت: قال النبي صلى الله عليه وسلم: «لَا تَسُبُّوا الْأَمْوَاتَ، فَإِنَّهُمْ قَدْ أَفْضَوْا إِلَى مَا قَدَّمُوا».
Translation
‘Ā’ishah (may Allah be pleased with her) reported: The Prophet (may Allah’s peace and blessings be upon him) said: "Do not curse the dead, for they have reached the result of what they have done."
Explanation
The Prophet (may Allah's peace and blessings be upon him) points out the prohibition of cursing the dead and speaking ill of their honor, and that this is considered among the bad manners. This is because they have reached the result of what they have done, be it righteous or evil. Moreover, cursing the dead does not reach them, but it only hurts the living ones.
Source: HadeethEnc.com