Is it permissible for the worshipper who missed a rak’ah and found the imam prostrating to say takbeer and then prostrate, or should he wait until the imam gets up?.
Praise be to Allaah.
If a person comes and finds the imam in a particular posture
he should join the imam in whatever posture he is in. So if he comes and
finds him prostrating, he should say takbeer and then prostrate, because the
Prophet (peace and blessings of Allaah be upon him) said: “Whatever you
catch up with, pray, and whatever you missed, make it up.” And because the
prostration that he does may be a cause of forgiveness of his sins, so he
should not miss out on that. Hence he should prostrate and then stand up
with his imam, but that is not counted as a rak’ah for him, because the
rak’ah can only be caught up with if one catches up with the bowing
(rukoo’). End quote.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him).