Dear brothers and sisters, may Allah rewards you all for the time and effort you put in to this site. Someone told me that when a person knows the Holy Qur'an by heart and doesn't get up on the third part of the night is like a sin upon him/her. Please, can you explain to me and make it clear for me and this person?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Late night prayers are reported from the Prophet (Sallallahu Alaihi wa Sallam) above all in the third part of the night when Allah "our Lord, the Blessed, the Superior, descends every night to the heaven of the world and says, 'Is there anyone who invokes Me (demand anything from Me), that I may respond to his invocation; is there anyone who asks Me for something that I may give (it to) him; is there anyone who asks My forgiveness that I may forgive him?" [ al-Bukhari ]. Late night prayer is one of the features of good pious people and it is one of the greatest causes for pleasing Allah, having a bright face, being well to do and it is a means to be obedient of Allah The Almighty.
As for saying that a person who memorizes Qur'an and does not read it or pray with it in the last third of the night commits a sin, this is not true because late night prayers are not obligatory. al-Bukhari and Muslim reported that "A man came to the Prophet (Sallallahu Alaihi wa Sallam) and said to him: 'O Prophet (Sallallahu Alaihi wa Sallam) of Allah inform me about what Allah made mandatory on me as far as the prayer is concerned?' The Prophet (Sallallahu Alaihi wa Sallam) said: "Five prayers along the day and the night". The man said: 'Am I obliged to perform other than that?' The Prophet (Sallallahu Alaihi wa Sallam) said: "No" .
But the person who does not perform late night prayers, especially the one who memorizes Qur'an misses a great benefit and might forget what he had memorized hence losing this great advantage that Allah has bestowed on him. We ask Allah to grant us and you the right guidance and help us perform that which pleases Him.
Allah knows best.