Arabic text
عن أبي ثَعْلَبَة الخُشَني رضي الله عنه قال: كان الناس إذا نزلوا منزلًا تفرقوا في الشِّعابِ والأَوْدِيَةِ، فقال رسول الله صلى الله عليه وسلم : «إن تَفَرُّقَكم في هذه الشِّعاب والأَوْدِية إنما ذلكم من الشيطان». فلم ينزلوا بعد ذلك منزلاً إلا انضم بعضهم إلى بعض.
Translation
Abu Tha‘labah al-Khushani (may Allah be pleased with him) reported: Whenever the people encamped during a journey, they would disperse in the mountain passes and valleys. When the Messenger of Allah (may Allah's peace and blessings be upon him) noticed this he said: "Your scattering about in the mountain passes and valleys is indeed from the devil." After that they kept close together whenever they set up camp.
Explanation
The people used to scatter in the valleys and mountain passes when encamped during a journey. The Prophet (may Allah's peace and blessings be upon him) informed them this is inspired by the devil in order to instill fear in them and lure the enemies of Allah (to attack them). After that whenever they set up their camp they would stay so close together that if a garment were spread over them it would cover them all.
Source: HadeethEnc.com