"Go forth, whether light or heavy," the meaning of which is: young and old.
Al-Kalbi said: This was when the Messenger of Allah (peace be upon him) summoned the people to Tabuk during intense heat and a time of hardship for the people. Some people disliked going out and began asking for permission to remain behind, from among those who had excuses and those who did not. He granted permission to whom he wished to grant it, and many of them remained behind without permission; so Allah (Blessed and Exalted is He) revealed:
﴿انفروا خفافا وثقالا﴾ قَالَ الْمَعْنى: شبَابًا وشيوخاً.
قَالَ الْكَلْبِيُّ: وَذَلِكَ حِينَ اسْتَنْفَرَ رَسُولَ اللَّهِ ﷺ النَّاسَ إِلَى تَبُوكَ فِي حرٍّ شَدِيدٍ، وَعُسْرَةٍ مِنَ النَّاسِ، فَكَرِهَ بَعْضُ النَّاسِ الْخُرُوجَ، وَجَعَلُوا يَسْتَأْذِنُونَ فِي الْمَقَامِ مِنْ بَيْنَ [ … ] وَمَنْ لَيْسَتْ بِهِ عِلَّةٌ؛ فَيَأْذَنُ لِمَنْ شَاءَ أَنْ يَأْذَنَ، وَتَخَلَّفَ كَثِيرٌ مِنْهُمْ بَغَيْرِ إِذْنٍ؛ فَأَنْزَلَ الله ﷿
