Does the prayer become invalid by saying the takbir at the word of an outsider?

Question

In a certain mosque the fard prayer is being performed in congregation; there are several storeys above, and in the upper part too people are joining the congregation. By misfortune the loudspeaker broke down, or the imam's voice did not reach upstairs. A man came down from above onto the staircase and called out: “Say the takbir — it cannot be heard upstairs!” One of those praying began saying the takbir in a low voice. The one who had called out said again: “Say the takbir loudly!

Answer

When someone who was not taking part in the prayer said, “Say the takbir,” — if, upon that, none of those praying said the takbir aloud immediately, but, for example, the imam was at that time occupied with recitation, and when he finished and went into ruku' one of those praying then said the takbir so that [the voice] would reach the worshippers upstairs as well — the prayer…

Source: Fatawa Mahmudiyya

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