It is narrated from Sayyiduna Abdullah bin Amr bin al-As (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When the imam is (sitting) in the sitting posture in the last rak'ah of his prayer, and his ablution is broken before he recites the tashahhud, his prayer is complete."
This is a weak hadith.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2822