Praise be to Allaah.  

It is makrooh to speak after ‘Isha’ prayer unless that is for a reason, or speaking about some good topic, because it was narrated in al-Saheehayn from Abi Barzah that the Messenger of Allaah (peace and blessings of Allaah be upon him) used to dislike sleeping before ‘Isha’ and speaking afterwards. Narrated by al-Bukhaari, 568; Muslim, 647. 

Al-Nawawi (may Allaah have mercy on him) said in his commentary: 

The scholars said: The reason for it being makrooh to sleep beforehand is that one risks missing the prayer by sleeping too deeply, or one risks missing the preferred and best time for the prayer, and lest the people take this matter lightly and sleep and miss the prayer in congregation. 

The reason why it is makrooh to speak afterwards is because that leads to people staying up late at night and there is the fear that sleep may then prevent them from praying qiyaam al-layl and making dhikr, or from praying Fajr during the permissible time or the best and preferred time. And because staying up late at night is a cause of laziness during the day with regard to religious duties, acts of worship and worldly interests. The scholars said: the kind of speech that is makrooh after ‘Isha’ is that which serves no interest. As for speech which serves a good interest, it is not makrooh, such as studying, telling stories of the righteous, speaking to a guest or to a bride to put them at their ease, a man speaking kindly to his wife and children or for a reason, speaking to travellers about protecting them or their luggage, speaking to reconcile people and intercede with them for a good purpose, enjoining what is good and forbidding what is evil, guiding people to do good, etc. All of these are not makrooh. There are saheeh ahaadeeth which speak of some of the things we have mentioned, and the rest come under the same heading.  What is meant by it being makrooh to speak after ‘Isha’ is speaking after praying ‘Isha’, not after the time for ‘Isha’ begins. The scholars are agreed that it is makrooh to speak after the prayer except speech in which there is goodness, as we have mentioned above. 

And he said in his book al-Adhkaar (p. 533): 

With regard to the ahaadeeth which indicate a concession allowing speaking about the things mentioned above, there are many such reports. For example, the hadeeth of Ibn ‘Umar in al-Saheehayn which says that the Messenger of Allaah (peace and blessings of Allaah be upon him) prayed ‘Isha’ at the end of his life, and when he had said the salaam he said: “Do you see this night of yours? One hundred years from now, no one who is on the face of the earth today will be left.” 

And it was reported in al-Saheehayn from Abu Moosa al-Ash’ari that the Messenger of Allaah (peace and blessings of Allaah be upon him) delayed the prayer until it was nearly midnight, then the Messenger of Allaah (peace and blessings of Allaah be upon him) went out and led them in prayer. When he finished his prayer he said to those who were with him: “Wait a moment. Receive the glad tidings that by the blessing of Allaah to you, no one has prayed at this hour except you.”  

In Saheeh al-Bukhaari it is narrated from Anas that they waited for the Prophet (peace and blessings of Allaah be upon him) and he came to them when it was nearly midnight and led them in prayer, meaning ‘Isha’. [Anas] said: Then he addressed us, saying, “The people have prayed then gone to bed, but you have still been in a state of prayer so long as you were waiting for the prayer.” 

Ibn ‘Abbaas (may Allaah be pleased with him) narrated that he stayed overnight in the house of his maternal aunt Maymoonah and said that the Prophet (peace and blessings of Allaah be upon him) prayed ‘Isha’, then he went in and spoke with his wife. 

‘Abd al-Rahmaan ibn Abi Bakr (may Allaah be pleased with him) narrated the story of his visitors and how he stayed away from them until he prayed ‘Isha’, then he came and spoke to them and to his wife and son. These two hadeeth are in al-Saheehayn, and there are many similar reports, more than can be listed, but what we have quoted here is enough, praise be to Allaah. 

To sum up the above: 

Speaking after ‘Isha’ prayer is permissible and is not makrooh if it serves a purpose. But if it serves no useful purpose then it is makrooh, but is not haraam, unless the speech itself is haraam such as backbiting and gossip. 

And Allaah knows best.