What is the reason for not mentioning the Tahajjud prayer and the Greeting of the Mosque among the sunnah prayers? What is the manner of performing Tahajjud, its number of rak'ahs, and its time? Is it a condition to sleep before it? Is perseverance in it necessary?

Tahajjud is one of the absolute supererogatory prayers. It is a voluntary prayer performed at night after sleeping, which is preferable, or a prayer performed after 'Isha, even before sleeping. Its time extends from after the 'Isha prayer until the true dawn. It is best performed in units of two rak'ahs. There is no limit to the number of rak'ahs for Tahajjud, and it is commendable to maintain it, even with a small number.

As for greeting the mosque (Tahiyyat al-Masjid), it is one of the Sunnahs prescribed for anyone who enters the mosque. Other prayers suffice for it if one does not intend Tahiyyat al-Masjid.