Yahya related to me from Malik that Ibn Shihab used to say, "Whena man catches the raka he says, 'Allah is greater' once, and thattakbir is enough for him."Malik added, "That is if heintended to begin the prayer by that takbir "Malik was askedabout a man who began with the imam but forgot the opening takbir andthe takbir of the ruku until he had done one raka. Then he rememberedthat he had not said the takbir at the opening nor in the ruku,so hesaid the takbir in the second raka. He said, "I prefer that he starthis prayer again, but if he forgets the opening takbir with the imamand says the takbir in the first ruku, I consider that enough for himif he intends by it the opening takbir."Malik said, aboutsome one who prayed by himself and forgot the opening takbir, "Hebegins his prayer afresh ."Malik said, about an imam whoforgot the opening takbir until he had finished his prayer, "I thinkthat he should do the prayer again, and those behind him, even if theyhave said the takbir."
App reference: Book 3, Hadith 23
USC-MSA web (English) reference: Book 3, Hadith 23
Arabic reference: Book 3, Hadith 170