بُلُوغُ الْمَرَامِ
Bulugh al-Maram 455
The Book of Prayer
حَدِيثَ أَبِي هُرَيْرَةَ - رضى الله عنه - فِي اَلصَّحِيحَيْنِ مَرْفُوعًا: { إِذَا قُلْتَ لِصَاحِبِكَ: أَنْصِتْ يَوْمَ اَلْجُمُعَةِ وَالْإِمَامِ يَخْطُبُ, فَقَدْ لَغَوْتَ } [1] .
This Hadith interprets the Marfu' Hadith [attributed to the Prophet (ﷺ)] of Abu Hurairah reported in the Sahihain: On Friday, when the Imam is delivering the Khutbah (religious talk), if you say to your companion to keep quiet, then indeed you have committed a sin (error/something senseless).
Sunnah.com reference: Book 2, Hadith 375