Musnad Ahmad 26:124

Muhammad ibn Ubayd narrated to us, Aban ibn Ishaq narrated to us, from al-Sabah ibn Muhammad, from Murrah al-Hamadani, from Abdullah ibn Mas’ud, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said one day: "Be shy of Allah, the Almighty and Majestic, with true shyness." He said: We said: "O Messenger of Allah, we are shy, and praise be to Allah." 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, let him abandon the adornment of the worldly life. Whoever does that, then he has been shy of Allah, the Almighty and Majestic, with true shyness."