فَقَالَ: عِبَادَ اللَّهِ! لَتُسَوُّنَّ صُفُوفَكُمْ أَوْ لَيُخَالِفَنَّ اللَّهُ بَيْنَ وُجُوهِكُمْ
Translation: He said: Servants of Allah, you must straighten your rows, or Allah will cause differences between your faces.
The Prophet ﷺ warned: “Servants of Allah, you must straighten your rows, or Allah will bring differences between your faces.” The warning makes clear that organizing the congregation is not an insignificant formality. The warning also calls attention to the danger of disunity and distance between hearts.
Both لَتُسَوُّنَّ and لَيُخَالِفَنَّ carry emphatic force, with an initial lām and the emphatic nūn. The first stresses the required action; the second strongly warns about the consequences of neglect.
Source: Muslim