Muhammad bin Thawban (may Allah have mercy on him) said that the Messenger of Allah (peace and blessings be upon him) said: "Whoever lifts a woman's covering and looks at her, the dower (mahr) has become obligatory."
(b) Muhammad bin Abd al-Rahman bin Thawban narrates from the Prophet (peace and blessings be upon him): "Whoever lifts a woman's head-covering and looks at her, the dower is due from him, whether he consummated the marriage or not."
This is munqati' (broken), and some of its narrators are not used as evidence.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصداق / 14487