I recently asked you this question "If we enter a mosque and there is very less time left for obligatory prayer (say < 2 min);Is it permissible to sit without praying Tahiyat al masjid or should we remain standing for these minutes"...But you directed meto other questions which did not answer mine.I want to know can we sit without praying tayihat al masjid if there is very less time for Iqamah Also after any fardh prayer is it allowed to say "Allahu Akbar" in unison.Note just "Allahu Akbar" once in unison

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

With regard to the permissibility of sitting before praying Tahiyyat Al-Masjid, according to the view that Tahiyyat Al-Masjid is recommended and not obligatory – as is the view of the majority of the scholars – it is permissible for you to sit whether there are 2 minutes or ten minutes remaining before the Iqaamah (the call that is made immediately prior to commencing the prayer). If the time that remains is not enough to perform Tahiyyat Al-Masjid, then you may remain standing until the Iqaamah is called, and this is better than sitting.

As regards saying Allaahu Akbar in unison after the prayer, raising the voice with Takbeer (saying Allaahu Akbar) and other Adhkaar after the prayer is confirmed in the Sunnah, as in the Hadeeth of Ibn ‘Abbaas “We used to know the end of the prayer of the Prophet  from the Takbeer.” [Al-Bukhari] Another Hadeeth reads: “Raising the voice with the mention of Allaah when people had performed the obligatory prayer was done at the time of the Prophet .”

This does not mean that they used to say Allaahu Akbar collectively in unison, but that each of them said it loudly without deliberately joining the others in unison. However, the majority of the scholars have interpreted the Hadeeth of Ibn ‘Abbaas to mean that the Prophet said that loudly only for some time in order to teach them the method of the Athkaar, and not that he always said that loudly. An-Nawawi said: “The preferred opinion is the Athkaar are to be done silently and not out loud except when one intends to teach the worshippers, in which case, one says it out loud according to the need of teaching.” For more benefit, please refer to Fatwa 83390.

Therefore, we do not know any evidence for saying the Takbeer in unison, and the Prophet said: “Whoever performs an act which is not in accordance with ours [our religion], it will be rejected from him.” [Muslim] Another narration reads: “Whoever introduces into this matter of ours (Islam) that which is not part of it, it will be rejected." [Al-Bukhari and Muslim]

Allaah Knows best.