Abdul Malik bin Aby عبد الملك بن أبي سليمان · The Book of the Fear Prayer

"The Messenger of Allah (ﷺ) offered the fear prayer during one of his battles. One group stood with him and another group faced the enemy. He led those who were with him in praying one rak'ah, then they went away and the others came, and he led them in praying one rak'ah. Then each group made up one rak'ah."