After the Isha prayer everyday, in a mosque of our community, some hadiths are read from different books (for a durarion of about 25 to 30 minutes) by a person, and we are few people who listen to it. Is it sinful if I or someone left the program (or a program like a 'dars' or 'bayan' of any Islamicially literate person) at the middle although he does not have something important to do?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger. 

Attending religious lessons, learning the ahaadeeth of the Prophet, sallallaahu ʻalayhi wa sallam, and patiently holding fast to it brings about great reward and goodness. The Muslim should not deprive himself of such rewards without a real excuse. Nevertheless, he bears no sin for leaving the hadeeth lesson even if he had no excuse, unless he only leaves because of disliking the ahaadeeth of the Prophet, sallallaahu ʻalayhi wa sallam, like the hypocrites, which is unlikely for the Muslim. Verily, disliking the divine revelation is a sin that renders one's good deeds worthless; Allaah, The Exalted, says (what means): {That is because they disliked what Allaah revealed, so He rendered worthless their deeds.} [Quran 47:9]

Abu Waaqid Al-Laythi reported that the Messenger of Allaah, sallallaahu ʻalayhi wa sallam, was sitting in the mosque along with some people when there came to him three persons; two of them stepped forward to the direction of the Messenger of Allaah, sallallaahu ʻalayhi wa sallam, and one of them went away. The two stood by the side of Messenger of Allaah, sallallaahu ʻalayhi wa sallam, and one of them found a space in his circle and he sat in that; and the other one sat behind him and the third one went away. When the Messenger of Allaah, sallallaahu ʻalayhi wa sallam, finished (his preaching), he said, “Should I not inform you about these three persons? One of them sought refuge with Allaah and Allaah gave him refuge, and the second one felt shy and Allaah showed kindness to his shyness (and so he was accommodated in that meeting), and the last one turned away, so Allaah turned away from him.” [Al-Bukhari and Muslim]

Ibn ʻAbd Al-Barr wrote:

“As for the last person (who left the gathering), the Prophet, sallallaahu ʻalayhi wa sallam, said about him, 'He turned away; so Allaah turned away from him.' This means that he turned away from carrying out a good deed and as a result Allaah denied him the rewards. It could also mean that he who turns away from such blessed gatherings is the one who suffers from hypocrisy and spiritual diseases of the heart because no one would ever turn away from such a blessed gathering with the Prophet, sallallaahu ʻalayhi wa sallam, except the one whose heart harbors hypocrisy and spiritual disease. If such a man had turned away from the gathering for a need or an excuse, then the Prophet, sallallaahu ʻalayhi wa sallam, would not have described his action as such. Indeed, when a person turns away from such gatherings for no excuse, then he subjects himself to the wrath of Allaah by His turning away from him. I implore Allaah to safeguard us from incurring His wrath by His grace and mercy.” [At-Tamheed]

Hence, we advise the brother who asked the question not to leave the religious lessons and thikr (remembrance of Allaah) assemblies except when he has an excuse.

Allaah knows best.