Musnad Ahmad 962:141

Ibrahim ibn Ishaq narrated to us, Ibn al-Mubarak and 'Attab narrated to us, they said: Abdullah—meaning Ibn al-Mubarak—narrated to us, Yahya ibn Ayyub informed us, from Ubayd Allah ibn Zahr, from Ali ibn Yazid, from al-Qasim, from Abu Umamah, from the Prophet, peace and blessings of Allah be upon him, who said: "There is no Muslim who looks at the beauties of a woman for the first time, then lowers his gaze, except that Allah creates for him an act of worship, the sweetness of which he finds."