The sitting between the two sajdah is a pillar while the Duaa is obligatory.

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It is more rewarding for him to join the congregation in the masjid but if the masjid is far away or this is for Tahajjud, this is permissible. Kindly refer to the following link(s): https://islamqa.info/en/2034

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The sunnah is to greet the muslims with salam – assalamu alaikum – when meeting them and when parting with them. However, whatever you have mentioned is permissible to be said.

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