Reminding the imam is to prompt him with the verse if he forgets it; as narrated by Ibn Umar, may Allah be pleased with him: "The Prophet, peace be upon him, prayed a prayer in which he was reciting, and he became confused. When he finished, he said to Abu ibn Ka'b: 'Yes.' He said: 'What prevented you from reminding me?'" This is found in Sunan al-Bayhaqi al-Kabir 3: 212, Musnad al-Shamiyeen 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 it was authenticated by the hafiz in Al-Talkhis 1: 284. And Ibn Mas'ud, may Allah be pleased with him, said: "If the imam becomes confused, 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 Majma' al-Anhar 1: 119.