Ali bin Sa'id (may Allah have mercy on him) said: I asked Imam Ahmad bin Hanbal (may Allah have mercy on him) about a person who omits the tashahhud, and he said: "He should repeat the prayer." I said: What is your view of the hadith of Sayyiduna Ali (may Allah be pleased with him), that is, 'man qa'ada miqdar al-tashahhud' (whoever sat for the length of the tashahhud)? He said: "That hadith is not sahih (authentic)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2825