Allah says,
حم
Ha Mim,
We have already discussed the individual letters at the beginning of Surah Al-Baqarah, and there is no need to repeat the discussion here.
In a Hadith recorded by Abu Dawud and At-Tirmidhi from one who heard it from the Messenger of Allah, it says;
إِنْ بُيِّتُّمُ اللَّيْلَةَ فَقُولُوا حم لَا يُنْصَرُون
When you go to bed tonight, recite Ha Mim, La Yunsarun."
Its chain of narrators is Sahih.
Allah says,
تَنزِيلُ الْكِتَابِ مِنَ اللَّهِ الْعَزِيزِ الْعَلِيمِ