Musnad Ahmad 30:3634

'Abd al-Samad narrated to us, Sa'id ibn 'Ubayd al-Hana'i narrated to us, 'Abdullah ibn Shaqiq narrated to us, Abu Hurayrah narrated: That the Messenger of Allah (peace and blessings of Allah be upon him) camped between Dajnan and 'Usfan. The polytheists said: "These people have a prayer that is dearer to them than their fathers and their firstborns"—meaning the 'Asr prayer—"so gather your resolve and incline upon them in a single assault." Then Gabriel (peace be upon him) came to the Prophet (peace and blessings of Allah be upon him) and commanded him to divide his companions into two halves. He was to pray with some of them, while the other group stood behind them, taking their precautions and their weapons. Then the others would come and pray with him, and those would take their precautions and their weapons, so that there would be one rak'ah for them with the Messenger of Allah (peace and blessings of Allah be upon him), and two rak'ahs for the Messenger of Allah (peace and blessings of Allah be upon him).