His prayer is invalid; because it is a mistake linguistically; because the comparative form "أفعلَ التفضيل" does not linguistically allow for elongation; and because "أكبار" is the plural of "كِبْر", which means drum, thus deviating from the meaning of "تكبير"; as narrated by Abdul Rahman bin Abza from his father, may Allah be pleased with him: "He prayed with the Messenger of Allah, peace be upon him, and he did not complete the takbir." This is found in Sunan Abu Dawood 1: 282, Musnad Ahmad 3: 406, Musnad Al-Tayalisi 1: 181, Sharh Ma'ani Al-Athar 1: 220, and Musnad Ibn Al-Ja'd 1: 33. See: Al-Tabeen 1: 114.