Translation:

81. Those who were left behind (in the Tabuk expedition) rejoiced in their inaction behind the back of the apostle of Allah: they hated to strive and fight with their goods and their persons in the cause of Allah: they said "Go not forth in the heat. Say "The fire of Hell is fiercer in heat." If only they could understand!

Notes (Tafseer)

1335. The Tabuk expedition had to be undertaken hurriedly in the heat of summer, because of a threat or fear of Byzantine invasion. They marched from Madinah about the month of September or October in the solar calendar.