All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
First of all, we say that the ruling on congregational prayer is an issue of difference of opinion among the scholars, some of them are not of the view that it is an obligation and some of them are of the opinion that it is an obligation in the mosque.
Some scholars are of the view that the congregational prayer is an obligation even if it is held at a place other than the mosque.
We have already mentioned their view in previous Fataawa.
Since you write to us in Arabic sometimes, so we refer you to some Fataawa in our Arabic site, which are as follows: 38639 and 191365.
Nonetheless, there is no difference of opinion that the prayer in the mosque is better for men as one gets the reward of congregational prayer, the reward of walking many steps to the mosque, waiting for the prayer, and the angels seek forgiveness for the person, and other virtues that are mentioned about congregational prayer in the mosque.
However, there is no doubt that walking upstairs and downstairs five times a day involves some hardship especially for the elderly or those who suffer from diseases in their legs or knees.
Therefore, if there is a considerable hardship, then there is no harm in staying behind from the congregational prayer in the mosque but one should be keen on performing it in congregation at home, as the reward of congregational prayer is not specific to the mosque.
The jurists who are of the view that congregational prayer is an obligation in the mosque mentioned a number of excuses that make it permissible not to attend the congregational prayer. These excuses revolve around physical or financial hardship.
For more benefit on congregational prayer, please refer to Fataawa 295357, 292729, 111020, 365824, 89364, 216331, and 362092, in our English site.
Allah knows best.