Abu al-Aliyah (may Allah have mercy on him) says that Sayyiduna Ibn Abbas (may Allah be pleased with them both) heard a man in the prayer saying 'Al-hamdu lillah' (Praise be to Allah) at the beginning of the tashahhud, so he (may Allah be pleased with them both) rebuked him and said: Begin the tashahhud with 'At-tahiyyat'.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2835