(a) Hammad says: I said to Ibrahim: Should I say 'Bismillah' (In the name of Allah) in the tashahhud? He said: Recite 'At-tahiyyat' (All verbal acts of worship). Hammad says: I said: Should I say 'Al-hamdu lillah' (All praise is for Allah)? He said: Say 'At-tahiyyatu lillah' (All verbal acts of worship are for Allah alone).
(b) Hammad says that when Sa'id bin Jubayr (may Allah have mercy on him) recited the tashahhud, he would also say 'Al-hamdu lillah' (All praise is for Allah).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2836