346 – ʿAlī ibn Abī Ṭālib (may Allah be pleased with him) reported: “The Messenger of Allah ﷺ said, ‘When any one of you arrives for the prayer and the imam is already in a given posture, he should do exactly as the imam is doing.’” Al-Tirmidhī transmitted it with a weak chain of narration. (1)

Status of the ḥadīth
The report is weak, yet it is strengthened by supporting evidence. Al-Talḫīṣ states: “Al-Tirmidhī related it through the narrations of ʿAlī and Muʿādh, both of which contain weakness and a break in the chain, and we know of no one who has traced it back to the Prophet through another route.”

Al-Shawkānī remarked in Nayl al-Awaṭār: “Although the ḥadīth is weak, it is corroborated by what Aḥmad (2618) and Abū Dāwūd (507) recorded from Ibn Abī Laylā on the authority of Muʿādh. Even though Ibn Abī Laylā did not hear directly from Muʿādh, Abū Dāwūd also narrated it through another path from ʿAbd al-Raḥmān ibn Abī Laylā, who said: ‘Our companions informed us that the Messenger of Allah ﷺ …’—and he mentioned the narration.”

Points derived from the ḥadīth
1 – It is recommended for the late-comer to join the imam immediately, in whatever posture he finds him—standing, bowing, prostrating, or otherwise.

2 – If he joins while the imam is standing or bowing, that entire rakʿah counts for him; but if he finds the imam sitting or prostrating, it does not.

The proof for the first situation is the Prophetic report narrated by Abū Dāwūd from Abū Hurayrah: “Whoever….”