[Machine] The Messenger of Allah ﷺ said, "When the Imam sits in the last Rak'ah of his prayer, then speaks before saying the Tashahhud, his prayer is invalidated, and this is a weak narration reported by Al-Qa'nabi from Al-Ifrigi."  

قَالَ رَسُولُ اللهِ ﷺ إِذَا قَعَدَ الْإِمَامُ فِي آخِرِ رَكْعَةٍ مِنْ صَلَاتِهِ ثُمَّ أَحْدَثَ قَبْلَ أَنْ يَتَشَهَّدَ فَقَدْ تَمَّتْ صَلَاتُهُ فَهُوَ حَدِيثٌ ضَعِيفٌ وَرَوَاهُ الْقَعْنَبِيُّ عَنِ الْإِفْرِيقِيِّ  

The Prophet ﷺ said: When the Imam completes the prayer and sits (for reciting tashahhud), and then becomes defiled (i.e. his ablution becomes void) before he speaks (to someone), his prayer becomes complete. And those who prayed behind him also complete the prayer. (Using translation from Abū Dāʾūd 617)   

"إذا قَضى الإِمامُ الصلاةَ وقَعَدَ فأحْدَثَ قبل أنْ يتكلمَّ فقدْ تمت صلاتُه ومَنْ كان خلفَهُ ممّنْ أتمَّ الصلاةَ".  

"إِذَا جَلَسَ الإِمامُ في آخرِ ركعةٍ، ثمَّ أحدثَ رجلٌ مِنْ خلفِهِ قبلَ أنْ يُسلمَ الإِمامُ فقد تمت صَلاتُهُ".