Muwatta Imam Malik Book of Jumu'a, Hadith 7

Yahya related to me from Malik from Ibn Shihab that Thalaba ibn Abi Malik al Quradhi informed him that in the time of Umar ibn al Khattab they used to pray on the day of jumua until Umar came out, and when Umar came out and sat on the mimbar and the muadhdhins called the adhan, they would sit and talk, and then when the muadhdhins were silent and Umar stood to give the khutba, they would pay attention and no-one would speak .
Ibn Shihab said, "The imam coming out stops prayer and his speaking stops conversation."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، عَنْ ثَعْلَبَةَ بْنِ أَبِي مَالِكٍ الْقُرَظِيِّ، أَنَّهُ أَخْبَرَهُ أَنَّهُمْ، كَانُوا فِي زَمَانِ عُمَرَ بْنِ الْخَطَّابِ يُصَلُّونَ يَوْمَ الْجُمُعَةِ حَتَّى يَخْرُجَ عُمَرُ فَإِذَا خَرَجَ عُمَرُ وَجَلَسَ عَلَى الْمِنْبَرِ وَأَذَّنَ الْمُؤَذِّنُونَ - قَالَ ثَعْلَبَةُ - جَلَسْنَا نَتَحَدَّثُ فَإِذَا سَكَتَ الْمُؤَذِّنُونَ وَقَامَ عُمَرُ يَخْطُبُ أَنْصَتْنَا فَلَمْ يَتَكَلَّمْ مِنَّا أَحَدٌ . قَالَ ابْنُ شِهَابٍ فَخُرُوجُ الإِمَامِ يَقْطَعُ الصَّلاَةَ وَكَلاَمُهُ يَقْطَعُ الْكَلاَمَ .