Since the latecomer to the congregational prayer does not count the rakʿah unless he has joined during the rukūʿ, some people believe that if they come after the rukūʿ there is no point in joining the prayer until the imām stands back up for the next rakʿah, even if it means standing there looking around for a long time while the congregation makes tashahhud!
Here’s why this is forbidden from a four-fold textual standpoint:
1) The generality of the ḥadīth which is found in al-Bukhārī and Muslim:
”The imām has only been appointed to be followed…”
2) And in one wording in al-Bukhārī (722):
”…So do not contraḍīct him…”
The one who stands there waiting for the imām to stand has not followed the imām, in fact he has contraḍīcted him!
3) In the same ḥadīth:
”…So if he makes takbīr (says: “Allāhu akbar”), then make takbīr…”
The one who stands there waiting for the imām to rise has not made the takbīrs that go with each movement, and thus has violated this order.
4) The clearest ḥadīth in the issue (if you were saying, “Well, that’s not clear enough for me…”):
”If you have come to the prayer and we are prostrating, then prostrate, but do not count it (as a rakʿah)…”
This ḥadīth was collected by Abū Dāwūd in his Sunan (893) from the narration of Abū Hurayrah (may Allāh be pleased with him). al-Albānī graded it ḥasan.
May Allāh grant us success in following the Messenger (ﷺ) in all our words and deeds, secret and open. And Allāh knows best.1
 Don’t just stand there!!