Musnad Ahmad 3538
Musnad of Abdullah bin Mas'ood
It was narrated that ‘Abdullah (رضي الله عنه) said: The Messenger of Allah (ﷺ) led us in offering the fear prayer. They stood in two rows; one row stood behind the Prophet (ﷺ) and the other stood facing the enemy. The Messenger of Allah (ﷺ) led the row that was behind him in praying one rak’ah, then they got up and left, and took the place of those who had been facing the enemy, and the others came and stood in their place. Then the Messenger of Allah (ﷺ) led them (the second group) in praying one rak'ah, then he said the tasleem and those people got up and prayed (a second rak'ah) by themselves. Then they said the tasleem and went and took the place of the ones who were facing the enemy, and that group came back to their place and prayed one rak'ah by themselves, then they said the tasleem.