Sunan Abi Dawud Book 4, Hadith 43
Did you pray in time of danger with the Messenger of Allah (ﷺ)?
AbuHurayrah replied: Yes. Marwan then asked: When? AbuHurayrah said: On the occasion of the Battle of Najd. The Messenger of Allah (ﷺ) stood up to offer the afternoon prayer. One section stood with him (to pray) and the other was standing before the enemy, and their backs were towards the qiblah. The Messenger of Allah (ﷺ) uttered the takbir and all of them too uttered the takbir, i.e. those who were with him and those who were facing the enemy. Then the Messenger of Allah (ﷺ) offered one rak'ah and the section that was with him also prayed one rak'ah. He then prostrated himself and those who were with him also prostrated, while the other section was standing before the enemy.
The Messenger of Allah (ﷺ) then stood up and the section with him also stood up. They went and faced the enemy and the section that was previously facing the enemy stepped forward. They bowed and prostrated while the Messenger of Allah (ﷺ) was standing in the same position. Then they stood up and the Messenger of Allah (may peace be upon) prayed another rak'ah and all of them bowed and prostrated along with him. After that the section that was standing before the enemy came forward and they bowed and prostrated, while the Messenger of Allah (ﷺ) remained seated and also those who were with him. The salutation then followed. The Messenger of Allah (ﷺ) uttered the salutation and all of them uttered it together. The Messenger of Allah (ﷺ) prayed two rak'ahs and each of the two sections prayed one rak'ah with him (and the other by themselves).
The prayer time of danger should be offered in the following way: The imam should stand (for prayer) and a section of the people should stand along with him. The other section should stand facing the enemy. The imam should perform bowing and prostrate himself along with those who are with him. He then should stand (after prostration) and, when he stands straight, he should remain standing. They (the people) should (in the meantime) complete their remaining rak'ah (i.e. the second one). They they should utter the salutation, and turn away while the imam should remain standing. They should go before the enemy. Thereafter those who did not pray should come forward and utter the takbir (Allah is most great) behind imam. He should bow and prostrate along with them and utter the salutation. Then they should stand and completed their remaining rak'ah, and utter the salutation.
Abu Dawud said: The tradition reported by Yahya b. Sa'id from al-Qasim is similar to the one transmitted by Yazid b. Ruman except that he differed with him in salutation. The tradition reported by 'Ubaid Allah is like the one reported by Yahya b. Sa'id, saying: He (the Prophet) remained standing.
We went out with the Messenger of Allah (ﷺ) to Najd. When we reached Dhat ar-Riqa at Nakhl (or in a valley with palm trees) he met a group of the tribe of Ghatafan. The narrator then reported the tradition to the same effect, but his version is other than that of Haywah. He added to the words "when he bowed along with those who were with him and prostrated" the words "when they stood up, they retraced their footsteps to the rows of their companions". He did not mention the words "their back was towards the qiblah".