Hadith No. 1
It was narrated that ‘Aa’ishah said: “The riders used to pass by us when we were with the Messenger of Allaah (peace and blessings of Allaah be upon him) in ihraam. When they came near, each of us would lower her jilbaab from her head over her face, and when they passed by we would uncover (our faces).” [Narrated by Abu Dawood, 1833; Ahmad, 24067]
This hadith has decisive proof that Niqab is Wajib, because the blessed wives of Prophet (salallaho alaihi wasalam) kept extreme care of covering themselves and did not reveal their faces infront of strangers, however when women are with their husbands and those with whom their Nikah is impermissible (being single that is) they can take off the face veil.
Hadith No.2
Volume 1, Book 8, Number 368: (Sahih Bukhari)
Narrated 'Aisha:
Allah's Apostle used to offer the Fajr prayer and some believing women covered with their veiling sheets used to attend the Fajr prayer with him and then they would return to their homes UNRECOGNIZED.
Even a child would know that If a woman returns home unrecognized (by being amongst men) then she must have covered her face!
Hadith No.3
Book 14, Number 2482: (Sunnan Abi Dawud)
Narrated Thabit ibn Qays:
A woman called Umm Khallad came to the Prophet (peace_be_upon_him) while she was veiled. She was searching for her son who had been killed (in the battle) Some of the Companions of the Prophet (peace_be_upon_him) said to her: You have come here asking for your son while veiling your face? She said: If I am afflicted with the loss of my son, I shall not suffer the loss of my modesty. The Apostle of Allah (peace_be_upon_him) said: You will get the reward of two martyrs for your son. She asked: Why is that so, Apostle of Allah? He replied: Because the people of the Book have killed him.
The Blessed women companions of Prophet (salallaho alaihi wasalam) did not even consider taking off their face veils in most painful situations (i.e. even at death of their children)
Hadith No.4
Aisha (RA) mentioned the women of Ansar, praised them and said good words about them. She then said: "When Surat an-Nur came down, they took the curtains, tore them and and made veils from them." [Sunann Abi Dawud Hadith No. 3577]
Imam Ibn Hajr al Asqalani (Rahimuhullah) in his Fath al Bari says that making veils here refers to covering the face!
Quran tells women to cover face if he can't understand Arabic it's his problem