Qasim bin Hassan said: I came to so-and-so bin Wadi'ah and asked him about the fear prayer, and he said: Go to Zayd bin Thabit (may Allah be pleased with him) and ask him. I came to Zayd bin Thabit (may Allah be pleased with him) and asked him. He said that the Prophet (peace and blessings be upon him) led the fear prayer: one row was behind him (peace and blessings be upon him) and the other row facing the enemy. He (peace and blessings be upon him) led them in one rak'ah. Then these went to the place of the other row, and those came to their place. He (peace and blessings be upon him) led them in one rak'ah, then gave the salam.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الخوف / 6050