The Messenger of Allah ﷺ said, "When the imam completes the prayer and sits, then breaks his ablution before speaking, his prayer is complete, as is the prayer of those behind him who have completed the prayer."
حدثنا أحمد بن يونس حدثنا زهير حدثنا عبد الرحمن بن زياد بن أنعم عن عبد الرحمن بن رافع وبكر بن سوادة
أن رسول الله ﷺ قال إذا قضى الإمام الصلاة وقعد فأحدث قبل أن يتكلم فقد تمت صلاته ومن كان خلفه ممن أتم الصلاة
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Its chain is daif. Tirmidhi transmitted it in the Book of Prayer, chapter on what has been said about a man who passes wind during the tashahhud, hadith no. 408, from the hadith of Abdul-Rahman bin Ziyad Afriqi, through him, and he declared it daif.Daraqutni said in 1/379, "Abdul-Rahman bin Ziyad is daif, not to be relied upon."See also 62, 514.