It is narrated from Sayyiduna Salamah bin al-Akwa' (may Allah be pleased with him): I was on a military expedition with Sayyiduna Abu Bakr (may Allah be pleased with him) in the time of the Messenger of Allah (peace and blessings be upon him), and our battle-cry was "Amit, amit!" (Kill, kill!).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / 13053