What is the ruling on combining the Dhuhr and Asr prayers, and the Maghrib and Isha prayers, due to forgetfulness, fatigue, and illness? And how many rak'ahs are there for each prayer when combined?

A Muslim must maintain the prayer at its prescribed time, for it is the pillar of Islam and the distinction between him and disbelief, and it is the first thing a servant will be held accountable for on the Day of Judgment. It does not cease to be obligatory upon him as long as he is conscious. The sick person prays according to his ability, standing, sitting, on his side, or lying down. Laziness in performing it is a characteristic of hypocrites who delay their prayers or pray them after their time, and woe to them. As for forgetting the prayer unintentionally, such as due to sleep, the Muslim prays it whenever he remembers it. It is permissible to combine two prayers (Dhuhr and Asr, Maghrib and Isha) by advancing or delaying them due to an excuse like travel or rain, and for the sick according to some jurists. As for Fajr, it is prayed at its prescribed time. Whoever misses a prayer due to an excuse, prays it whenever he remembers it. The number of rak'ahs for combined prayers is the same as the full number of rak'ahs for the prayer, unless it is during travel, in which case the four-rak'ah prayer is shortened to two rak'ahs.