Reuters doesn't give the full quote or the reference, but for those who want to read it, here it is (in a different translation):
CHATTANOOGA, Tenn. (Reuters) - Hours before the Tennessee shooting that killed five U.S. servicemen, the suspected gunman texted his close friend a link to a long Islamic verse that included the line: "Whosoever shows enmity to a friend of Mine, then I have declared war against him."
Mohammod Youssuf Abdulazeez
His friend thought nothing of it at the time, but now wonders if it was a clue to Thursday's rampage in Chattanooga, which has re-ignited concerns about the radicalization of young Muslim men.
"I didn't see it as a hint at the time, but it may have been his way of telling me something," the friend told Reuters on Saturday. He requested anonymity for fear of a backlash.
The suspect, Mohammod Youssuf Abdulazeez, a 24-year-old Kuwaiti-born naturalized U.S. citizen, was killed in a gunfight with police. The FBI is investigating the attack as an act of terrorism, but said it was premature to speculate on the gunman's motive.
While a firm connection between the suspect and radical Islam has not been established, the shooting follows a series of attacks or thwarted attacks in the United States and other countries by Muslims claiming to be inspired by Islamic State or other militant groups. (Continue Reading.)
I'm not sure which part was most important for Abdulazeez, but there are a few possibilities.
First, there is the portion quoted by Reuters about Allah declaring war on someone who shows enmity towards a Muslim. Since the U.S. military has killed numerous Muslims in various wars, Abdulazeez was clearly convinced that Allah had declared war on the U.S. military. When Allah declares war, Muslims must fight.
Second, Allah says that the most beloved things a Muslim can approach him with are the deeds commanded by Allah. If Abdulazeez regarded Allah's commands to "Fight those who do not believe in Allah" (Qur'an 9:29) or to "Fight those of the unbelievers who are near to you and let them find in you hardness" (9:123) as commands directed towards Muslims in general, he may have believed that killing U.S. Marines was straightforward obedience to clear commands.
Third, Allah points out that a Muslim can come even closer to him by going beyond the performance of obligatory deeds. Hence, Abdulazeez may have believed that he was going beyond the call of duty by attacking U.S. Marines, and that his actions would get him special honor.
Fourth, Allah declares that he will come so close to Muslims who perform supererogatory deeds (deeds that go beyond the call of duty), he will actually become their sense of sight and their sense of hearing. If Abdulazeez had a spiritual hunger, he may have longed to be so close to Allah that Allah becomes his senses.
Fifth, if a Muslim performs supererogatory deeds, Allah promises to protect the Muslim who asks for protection. The hadith may therefore have been a request for divine protection.
Of course, Abdulazeez may have meant more than one of the above possibilities. But since he threw his life away waging jihad for Allah, I guess we'll never know.