His prayer is invalidated; however, if he opens it up to his imam, it does not invalidate the prayer of the opener or the imam, even if he opens it up after reading the amount that makes the prayer valid, or if he moves to another verse, according to the correct opinion. Although it is preferable to refrain from opening it here; for Ibn Umar, may Allah be pleased with him, said: "The Prophet, peace be upon him, prayed a prayer in which he was reading, and he got confused. When he finished, he said to Ubayy ibn Ka'b: 'Yes.' He said: 'What prevented you from opening up to me?'" This is found in Sunan al-Bayhaqi al-Kabir 3: 212, Musnad al-Shamiyin 1: 437, and Al-Mu'jam al-Kabir 12: 313, and its narrators are trustworthy as mentioned in Majma' al-Zawa'id 1: 169. See: I'la al-Sunan 5: 56. And Ali, may Allah be pleased with him, said: "If the imam asks you for help, then help him," in Musannaf Ibn Abi Shaybah 1: 417, and the hadith was authenticated by the Hafiz in al-Talkhis 1: 284. And Ibn Mas'ud, may Allah be pleased with him, said: "If the imam stumbles, do not respond to him, for it is speech." Al-Haythami said in Majma' al-Zawa'id 2: 69: "It was narrated by Al-Tabarani in Al-Kabir, and its narrators are the narrators of the Sahih." See: Rad al-Muhtar 1: 418, and in Majma' al-Anhar 1: 119.