Prayer becomes invalid due to speech, whether it is intentional or by mistake; because engaging in what is not suitable during prayer invalidates it, whether done intentionally or forgetfully, whether it is little or much: like eating and drinking. It is narrated from Muawiya ibn Al-Hakam, may Allah be pleased with him, that the Prophet, peace be upon him, said: ((This prayer is not suitable for any human speech, it is only for glorifying Allah, declaring His greatness, and reciting the Quran)), in Sahih Muslim 1: 381, Sahih Ibn Khuzaymah 2: 35, and Sahih Ibn Hibban 6: 23. And from Zaid ibn Arqam, may Allah be pleased with him, who said: ((We used to speak during prayer, a man would talk to his companion while he was next to him in prayer until the verse was revealed: (And stand before Allah with devotion) Al-Baqarah: from verse 238, so we were commanded to be silent and forbidden from speaking)), in Sahih Muslim 1: 383. See: Al-Bahr 2: 8-9, Fath Bab Al-Inayah 1: 301, and Radd Al-Muhtar 1: 414.