His imam said three takbeers in the funeral prayer, and forgot the fourth; should he follow him?

In the answer to
question no. 159418 we stated
that the number of takbeers in the funeral prayer is four, all of which are
pillars or essential parts of the prayer. 

Based on that, if
the imam says only three takbeers by mistake, he should not be followed, and
it is essential to alert him to that. If he then says the fourth takbeer,
his prayer is valid, but if he does not do that, then the people praying
behind him have to complete the fourth takbeer, then say the salaam. 

Al-Bahooti (may
Allah have mercy on him) said: If a worshipper omits one of the four
takbeers deliberately, his prayer becomes invalid, because he has omitted an
obligatory part deliberately; hence it becomes invalid, as in the case of
all other prayers. If he omitted it by mistake, he should say it; this is
similar to the case if he says the salaam in an obligatory prayer before
completing it by mistake – he should (correct his mistake) if it is soon
afterwards, and his prayer is still valid. 

End quote from
Daqaa’iq Ooli an-Nuha, 1/362 

Al-Bukhaari
narrated, in the chapter on saying takbeer in the funeral prayer: Humayd
said: Anas (may Allah be pleased with him) led us in offering the (funeral)
prayer, and he said takbeer three times, then he said the salaam. It was
pointed out to him, so he turned to face the qiblah, then he said the fourth
takbeer, then he said the salaam. 

Al-Haafiz (may
Allah have mercy on him) said: ‘Abd ar-Razzaaq narrated from Ma‘mar, from
Qataadah, from Anas, that he said takbeer three times in the funeral prayer,
then he ended it by mistake. They said: O Abu Hamzah, you only said takbeer
three times. He said: Get back into your rows. So they did so, and he said
the fourth takbeer.

End quote from
Fath al-Baari, 3/202 

Shaykh Ibn
‘Uthaymeen (may Allah have mercy on him) was asked: If the imam says the
takbeer three times (in the funeral prayer) then says the salaam, what
should the one who is praying behind him do? He replied: He has to alert him
to that. 

Question: What if
he is not alerted to it? 

Shaykh: If he is
not alerted to it, then the one who is praying behind him should say the
takbeer, and thus the communal obligation will have been fulfilled. 

End quote from
ash-Sharh al-Kaafi. 

And Allah knows
best.