Musnad Abu Ya'la al-Mawsili 152:308

Abu Khaythama narrated to us, Jarir narrated to us, from ‘Ata’ ibn al-Sa’ib, from Abu ‘Ubayda, from ‘Abdullah, who said: Beware of these testimonies, that a man should say, "So-and-so was killed as a martyr." For a man may fight out of tribal zeal, and he may fight while being bold-hearted, and he does not know for what he is fighting, and he may fight for the sake of the world. I will tell you about that: The Messenger of Allah (ﷺ) sent a group, a military detachment, and they did not remain for long before the Messenger of Allah (ﷺ) stood up, praised Allah and extolled Him, and then said: "Your brothers met the enemy and they were cut off, and not a single man among them escaped. They met their Lord, and they said: 'Our Lord, inform our people that we have become pleased and [He] has become pleased with us.' And I am their messenger to you, that they have become pleased and [He] has become pleased with them." So, for the likes of these, bear witness.