(O ye who believe! What aileth you that when it is said unto you: Go forth in the way of Allah…) [9:38].
This verse was revealed to prompt the believers to take part in the Battle of Tabuk. When the Messenger of Allah, Allah bless him and give him peace, returned from al-Ta'if and the Battle of Hunayn, he gave orders to attack the Byzantines. This happened at a time when people were living in straitened circumstances, the land was suffering from drought; it was very hot and it was time to reap the dates which were by then ripe. People found it very hard to conquer the Byzantines; they preferred to stay at home and enjoy the shade. The idea of going out to fight was very hard to accept. When Allah knew people's slackness, He revealed this verse.
Asbab Al-Nuzul by Al-Wahidi , trans. Mokrane Guezzou