His prayer is invalid; because it is a question, and if he does it intentionally, it is disbelief; due to the doubt in arrogance; 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," in Sunan Abi 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.