Musannaf Ibn Abi Shayba 21:605

Waki' narrated to us, from Sufyan, from Zubayd, from Murrah, who said: Abdullah said: "Whoever among you is too cowardly to fight the enemy, or too weak to endure the night in worship, or too stingy with his wealth to spend it, then let him frequently say: 'Subhan Allah' (Glory be to Allah), 'Al-hamdulillah' (Praise be to Allah), 'La ilaha illallah' (There is no god but Allah), and 'Allahu Akbar' (Allah is the Greatest)."