Musannaf Ibn Abi Shayba 3:2411

Ibn ‘Ulayyah narrated to us, from Maymun ibn Hamzah, from Ibrahim, from Ibn Mas‘ud, regarding the prompting of the Imam: "It is merely speech that he prompts him with." He said: And Ibrahim said: "I do not care whether I prompted him (laqantuhu) or said: 'O Kathir' (or 'Kabirah')."