Sayyiduna Nu'aym bin Hammar (may Allah be pleased with him) narrates: A man asked the Prophet (peace and blessings be upon him): "Which of the martyrs is most excellent?" The Prophet (peace and blessings be upon him) said: "Those who, when they stand in the battle line, do not turn their faces away until they are killed. These people will move about in the highest chambers of Paradise, and their Lord will smile at them; and when the Lord smiles at a servant in this world, no reckoning will be taken from that servant."
Ahmad and Abu Ya'la recorded it. Abu Ya'la's wording is: It is reported from Sayyiduna Nu'aym bin Hammar (may Allah be pleased with him) that he heard the Prophet (peace and blessings be upon him) when a man came to him and said: "Which of the martyrs is most excellent?" The Prophet (peace and blessings be upon him) said: "Those who take part in the front row." The rest of the narration is as before.
Al-Tabarani recorded something similar in al-Mu'jam al-Kabir and al-Mu'jam al-Awsat. The narrators of Ahmad and Abu Ya'la are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الجهاد / 9513