Lowering One's Gaze
Narrated Abdullah ibn Abbas (radi Allahu anhu): "Al-Fadl bin Abbas rode behind Allah's Messenger (sal Allahu alaihi wa sallam) as his companion rider on the back portion of his she-camel on the day of Nahr (slaughtering of sacrifice, 10th Dhul-Hijja) and Al-Fadl was a handsome man. The Prophet (sal Allahu alaihi wa sallam) stopped to give the people verdicts (regarding their matters). In the meantime, a beautiful woman from the tribe of Khatham came, asking the verdict of Allah's Messenger. Al-Fadl started looking at her as her beauty attracted him. The Prophet (sal Allahu alaihi wa sallam) looked behind while Al-Fadl was looking at her; so the Prophet (sal Allahu alaihi wa sallam) held out his hand backwards and caught the chin of Al-Fadl and turned his face (to the other side) in order that he should not gaze at her. She said, 'O Allah's Messenger! The obligation of performing Hajj enjoined by Allah on his worshippers has become due (compulsory) on my father who is an old man and who cannot sit firmly on the riding animal. Will it be sufficient that I perform Hajj on his behalf?' He said, 'Yes.'" [Sahih Bukhari]
This hadith gives several important rulings. One is that the Prophet (sal Allahu alaihi wa sallam) acted himself, and made other men act, on Allah's orders to lower their gazes. [Quran 24: 27-29] We see in this hadith, that the Messenger of Allah (sal Allahu alaihi wa sallam) listened to the woman's question and answered it while not looking at her. He also turned the face of his cousin to the side who had been staring at the woman's beautiful face. He (sal Allahu alaihi wa sallam) did not ask the woman to cover her face.
This incident took place during Hajj, when even those women who normally cover their faces are required to leave their faces uncovered. Other ahadith show that the Sahabiyat would go for Fajr and Isha prayers in the Prophet’s masjid with their faces uncovered and not be recognized on account of the darkness, not because their faces were covered.
This topic, of whether women need to cover the face or can leave it uncovered, has evidence on both sides of the argument. So long as evidence from the Quran and Sunnah exists for an opinion, one must refrain from falling into the devil's trap of involving oneself in debate over it. It would be much more useful to talk about Allah (subhana wa ta'ala) and the Hereafter to the Kuffar (disbelievers) or Fasiqoon (those acting in clear contradiction to what is well known to be a part of Islam, such as avoiding alcohol).
From this hadith we also learn that it is permissible to perform Hajj on behalf of those who are too old or sick to undertake the journey themselves.