The wisdom behind the prohibition of separating and not joining together in gatherings (Explanation of the statement of the scholars:

The Shaykh : The wisdom behind such a command is known by one who has studied Islam in the Book of Allah and in the Sunnah of the Messenger of Allah (peace and blessings be upon him); he sees it clearly and manifestly. Indeed, one who has no knowledge of the Book and the Sunnah at all may also know it, provided he has some study in what is called today the science of psychology. This is because the Shari'ah established, before such a science known as psychology was recognized among people today, explicitly in many texts: that difference in the outward is a significant cause of difference in the inward. This means that all Muslims must care for repairing their outward just as they care for repairing their inward, and they must not be lazy or neglect the care of repairing the outward on the pretext that the criterion is only the repair of the inward. This is because Islam linked the two with a strong link and did not separate the soundness of the outward from the soundness of the inward; rather, it made them twins, each helping the other toward completeness and perfection.
Among the strongest evidence for this, known in the authentic Sunnah agreed upon byThe Shaykh : al-Bukhari and Muslim, where they narrated from al-Nu'man ibn Bashir (may Allah be pleased with him and his father), who said: The Messenger of Allah (peace and blessings be upon him and his family) said: .
The point of relevance is in what follows: .
Therefore, the soundness of the body is through the soundness of the heart, and the soundness of the heart is through the soundness of the body, and between them is what is called a constant movement; each helps the other. So it is a grave error, a clear and manifest error, what we hear from many young people who neglected performing many of the obligatory duties and obligations, such as prayer and fasting, for example. When they are asked, "Do you not pray? Do you not fast?" their answer is: "The criterion, my brother, is what is in the heart, and religion is dealing with people," they claim, "and I do not cheat, I do not betray, I do not lie," and so on with words like these. This is a terrible ignorance of this legal reality first, and then of the psychological reality second, that the outward affects the inward in a way that the man who does not care for repairing his outward does not feel.
Naturally, I do not mean by repairing the outward the repair of clothing and housing and the like, although the least that can be said about that is: Islam does not prohibit it. But I mean by it the repair of the actions that issue from a person, which the Shari'ah has encouraged, whether the encouragement is within the limits of obligatory duties and obligations or within the limits of recommended practices and recommended acts. These are the actions that are the cause for strengthening the heart and repairing it.
Look, for example, at how the Prophet (peace and blessings be upon him) gave a practical example where this example is realized negatively or positively five times every day. How the Prophet (peace and blessings be upon him) clarified this previous meaning in the hadith of al-Nu'man ibn Bashir, that the soundness of the outward affects the soundness of the inward, and vice versa completely. That is because it was part of the Sunnah of the Prophet (peace and blessings be upon him), which he cared about, when the prayer was established in the mosque, that he would not proceed with the takbir of ihram until he ordered the straightening of the rows. And his command was in many phrases, of which what concerns us now is his saying (peace and blessings be upon him): Straightening the rows is an outward action, and this is an example of what I meant previously by repairing the outward, as an example that follows the example of sitting in a gathering or in the mosque joined together, not separated. This is an outward action, but it has an effect on the inward, on the heart, a profound and great effect. And therefore he (peace and blessings be upon him) said: Meaning: