Musnad Ahmad 26:402

Rawh narrated to us, Hammad narrated to us, ‘Ata’ ibn al-Sa’ib informed us, from Abu ‘Ubaydah, from ‘Abdullah ibn Mas’ud, who said: Beware of saying: "So-and-so died a martyr," or "So-and-so was killed a martyr." For a man may fight to obtain spoils, and he may fight to be mentioned, and he may fight to have his position seen. If you must bear witness, then bear witness for the group whom the Messenger of Allah (peace and blessings of Allah be upon him) sent on an expedition, and they were killed. They said: "O Allah, convey to our Prophet (peace and blessings of Allah be upon him) on our behalf that we have met You, and we are pleased with You, and You are pleased with us."