If a person starts telling you, whether you are alone or in the company of others, something that you already knew very well, you should pretend as if you do not know it. Do not rush to reveal your knowledge or to interfere with the speech. Instead, show your attention and concentration.
The honourable Tābi‘ī Imām ‘Atā Ibn Abī Rabāh said, “A young man would tell me something that I may have heard before he was born. Nevertheless, I listen to him as if I had never heard it before.”
Khālid Ibn Safwān al-Tamīmī, who frequented the courts of the two Khalīfas; ‘Umar Ibn ‘Abd al-‘Azīz and Hisham Ibn ‘Abd al-Mālik, said, “If a person tells you something you have heard before, or news that you already learnt, do not interrupt him to exhibit your knowledge to those present. This is rude and ill-mannered.”
The honourable Imām ‘Abdullāh Ibn Wahab al-Qurashī al-Masrī, a companion of Imām Mālik, al-Layth Ibn Sa‘d and al-Thawrī, said, “Sometimes a person would tell me a story that I have heard before his parents had wed. Yet I listened as if I have never heard it before.”
Ibrāhīm Ibn al-Junayd said, “A wise man said to his son, ‘Learn the art of listening as you learn the art of speaking.’” Listening well means maintaining eye contact, allowing the speaker to finish the speech and restraining your urge to interrupt his speech.
Al-Hāfiz al-Khatīb al-Baghdādī said in a poem:
Never interrupt a talk
Though you know it inside out
Source: Islamic Manners, by Shaykh Abd al-Fattah Abu Ghuddah