Musnad Ahmad 33:812

'Affan narrated to us, Abu 'Awanah narrated to us, Abu Bishr narrated to us, from Sulayman ibn Qays, from Jabir ibn 'Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) fought [the tribe of] Muharib Khasfah at Nakhla. They saw an opportunity [to attack] the Muslims. A man from among them, called Ghawrath ibn al-Harith, came until he stood over the head of the Messenger of Allah (peace and blessings of Allah be upon him) with a sword and said: "Who will protect you from me?" He said: "Allah." The sword fell from his hand, and the Messenger of Allah (peace and blessings of Allah be upon him) took it and said: "Who will protect you from me?" He said: "Be the best of those who take [retribution]." He said: "Do you bear witness that there is no god but Allah?" He said: "No, but I will make a covenant with you that I will not fight you, nor will I be with a people who fight you." So he let him go. He said: He went to his companions and said: "I have come to you from the best of people." When it was the time of Dhuhr or 'Asr, he led them in the Prayer of Fear (Salat al-Khawf). The people were in two groups: one group facing their enemy, and one group praying with the Messenger of Allah (peace and blessings of Allah be upon him). He prayed two rak'ahs with the group that was with him, then they departed and took the place of those who were facing their enemy. Then those [others] came, and the Messenger of Allah (peace and blessings of Allah be upon him) prayed two rak'ahs with them. Thus, the people had two rak'ahs each, and the Messenger of Allah (peace and blessings of Allah be upon him) had four rak'ahs.