Musannaf Ibn Abi Shayba 33:95

Abdullah ibn Numayr narrated to us, from Muhammad ibn Ishaq, from al-Sabah ibn Muhammad al-Ahmasi, from Murrah al-Hamadani, from Abdullah ibn Mas’ud, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Be shy of Allah with true shyness." He said: We said: "O Messenger of Allah (may Allah bless him and grant him peace), we are indeed shy." He said: "It is not that. Rather, whoever is shy of Allah with true shyness, let him guard the head and what it contains, and let him guard the stomach and what it gathers, and let him remember death and decay. And whoever desires the Hereafter leaves the adornment of the world. Whoever does that has been shy of Allah with true shyness."