Musannaf Ibn Abi Shayba 21:460

Muhammad ibn Ubayd narrated to us, Wā’il ibn Dāwūd narrated to us, he said: I heard al-Hasan al-Baṣrī narrating. He said: While a man was asleep, he saw in a dream a caller calling out in the sky, "O people, take your weapon of alarm." So the people went and took up weapons, to the point that a man would come and have nothing with him but a staff. Then a caller from the sky called out, "This is not your weapon of alarm." A man from the earth asked, "What is our weapon of alarm?" He said, "Glory be to Allah (Subḥān Allāh), and praise be to Allah (al-Ḥamdulillāh), and there is no god but Allah (lā ilāha illā Allāh), and Allah is the Greatest (wa Allāhu Akbar)."