Setting
When one of you joins the prayer and finds the Imām in a certain position, he should do what the Imām is doing
When one of you joins the prayer and finds the Imām in a certain position, he should do what the Imām is doing
‘Ali ibn Abi Tālib and Mu‘ādh ibn Jabal (may Allah be pleased with both of them) reported that the Prophet (may Allah's peace and blessings be upon him) said: "When one of you joins the prayer and finds the Imām in a certain position, he should do what the Imām is doing."
[Sahih/Authentic.] [At-Tirmidhi]
Translations
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If one of you joins the congregational prayer and finds the Imām in one of the four positions of prayer: standing up, bowing, prostrating, or sitting down, he has to do the same as the Imām immediately without waiting for the Imām to stand up like what laymen do.Benefits
It is legitimate for a person coming late to prayer to join the Imām at any part of the prayer, with no difference between Rukū‘, prostration, or sitting.
Catching up a Rak‘ah with the Imām is realized by catching up its Rukū‘, as indicated by other Hadīths.