They went to Israa Qunfud ذَهَبُوا إِسْرَاءَ قُنْفُذٍ‏.‏

That is, their going at night was like a hedgehog that only goes at night
---------------------------------------------------------------------
The proverb "They traveled under the cover of night" is used to describe someone who travels or journeys at night, as the hedgehog is known for its nocturnal activity.
Some of the closest English equivalents are:
"They traveled under the cover of night."
This is the closest direct equivalent.
"They moved by night."
"They set out after dark."
"Like creatures of the night."
"They traveled by moonlight."
The closest equivalent to the Arabic proverb is:
"They traveled under the cover of night."
This preserves the image of nocturnal travel inherent in the Arabic proverb.
--------------------------------------------------------------------------------------------------
أي كان ذهابهم ليلا كالقنفذ لا يَسْرِي إلا ليلا‏.‏