Battle of Tabuk
Mujahid and several others said, "This Ayah was revealed concerning the battle of Tabuk. They left for that battle during a period of distress. It was a year with little rain, intense heat and scarcity of supplies and water." Qatadah said, "They went to Ash-Sham during the year of the battle of Tabuk at a time when the heat was intense. Allah knew how hard things were, and they suffered great hardship. We were told that two men used to divide a date between themselves. Some of them would take turns in sucking on a date and drinking water, then give it to another man to suck on. Allah forgave them and allowed them to come back from that battle." Ibn Jarir reported that `Abdullah bin `Abbas said that `Umar bin Al-Khattab was reminded of the battle of distress (Tabuk) and `Umar said, "We went with the Messenger of Allah ﷺ in the intense heat for Tabuk. We camped at a place in which we were stricken so hard by thirst that we thought that our necks would be severed. One of us used to go out in search of water and did not return until he feared that his neck would be severed. One would slaughter his camel, squeeze its intestines and drink its content, placing whatever was left on his kidney. Abu Bakr As-Siddiq said, `O Allah's Messenger! Allah, the Exalted and Most Honored, has always accepted your invocation, so invoke Allah for us.' The Prophet said,
«تُحِبُّ ذَلِكَ؟»
(Would you like me to do that) Abu Bakr said, `Yes.' The Prophet raised his hands and did not put them down until rain fell from the sky in abundance. It rained and then stopped raining for a while, then rained again, so they filled their containers. We went out to see where the rain reached and found that it did not rain beyond our camp."' Ibn Jarir said about Allah's statement,
لَقَدْ تَابَ الله عَلَى النَّبِىِّ وَالْمُهَـجِرِينَ وَالاٌّنصَـرِ الَّذِينَ اتَّبَعُوهُ فِى سَاعَةِ الْعُسْرَةِ
(Allah has forgiven the Prophet, the Muhajirin and the Ansar who followed him in the time of distress,) meaning "With regards to expenditures, transportation, supplies and water,
مِن بَعْدِ مَا كَادَ يَزِيغُ قُلُوبُ فَرِيقٍ مِّنْهُمْ
(after the hearts of a party of them had nearly deviated,) away from the truth, thus falling prey to doubting the Messenger's religion because of the distress and hardships they suffered during their travel and battle,
ثُمَّ تَابَ عَلَيْهِمْ
(but He accepted their repentance.) He directed them to repent to their Lord and renew their firmness on His religion,
إِنَّهُ بِهِمْ رَءُوفٌ رَّحِيمٌ
(Certainly, He is unto them full of kindness, Most Merciful.)"
Tafsir Ibn Kathir (abridged, English).