The Prophet ﷺ said: Stand close together in your rows, bring them near one another, and stand neck to neck, for by Him in Whose hand my soul is, I see the devil coming in through openings in the row just like a small black sheep. (Using translation from Abū Dāʾūd 667)
أَنَّ النَّبِيَّ ﷺ قَالَ رَاصُّو صُفُوفَكُمْ وَقَارِبُوا بَيْنَهَا وَحَاذُوا بِالْأَعْنَاقِ فَوَالَّذِي نَفْسُ مُحَمَّدٍ بِيَدِهِ إِنِّي لَأَرَى الشَّيَاطِينَ تَدْخُلُ مِنْ خَلَلِالصَّفِّ كَأَنَّهَا الْحَذَفُ وَقَالَ عَفَّانُ إِنِّي لَأَرَى الشَّيْطَانَ يَدْخُلُ