Musnad Abu Ya'la al-Mawsili 189:1

Daud ibn Rashid narrated to us, Ismail ibn Ayyash narrated to us, from Buhayr ibn Sa’d, from Khalid ibn Ma’dan, from Kathir ibn Murrah, from Nu’aym ibn Hammar, that he heard the Prophet, peace and blessings of Allah be upon him, and a man came to him and said, "Which of the martyrs is the best?" He said, "Those who meet in the rank, so they do not turn their faces until they are killed. Those are the ones who will recline in the high chambers of Paradise. Your Lord will laugh at them, and if He laughs at a person, there is no reckoning upon him."