Alerting another worshipper to a mistake in prayer

Question

If, while praying, one becomes aware of a lapse (sahw) of another worshipper standing beside or near him, which that person himself has not noticed — for example, he prayed five rak'as instead of four — is it necessary to inform him or not?

Answer

It is necessary.

Source: Fatawa Mahmudiyya

Preview. The full text opens with JavaScript enabled.