Bukhari, Volume 1, Book 4, Number 148:
The wives of the Prophet used to go to Al-Manasi, a vast open place (near Baqia at Medina) to answer the call of nature at night. 'Umar used to say to the Prophet "Let your wives be veiled," but Allah's Apostle did not do so. One night Sauda bint Zam'a the wife of the Prophet went out at 'Isha' time and she was a tall lady. 'Umar addressed her and said, "I have recognized you, O Sauda." He said so, as he desired eagerly that the verses of Al-Hijab (the observing of veils by the Muslim women) may be revealed. So Allah revealed the verses of "Al-Hijab" (A complete body cover excluding the eyes).
It is interesting to see how the prophet's wives would go to a certain designated spot in Medina to "answer the call of nature." What's more interesting is that Muhammad's companions knew where his wives would "answer the call of nature."
Furthermore, it is interesting to see that the wishes of Muhammad's followers could effectively influence Muhammad.
As it is written here, Umar wanted the Prophet to veil his wives, but the prophet only did so once Umar recognized one of his wives answering the call of nature in an open field.
The questions that must be answered are as follows:
1) How did Umar know that the verses of Al-Hijab would be revealed if he recognized one of the prophet's wives answering the call of nature?
a) How many other surahs were also anticipated in a cause and effect type situation where the Muhammad's followers would perform an action so that he could reveal a verse?
2) Did the angel Gabriel inspire Umar to say those words so that the angel would then inspire Muhammad with more of Allah's words, especially concerning the Hijab?
3) Why did Muhammad's followers know where his wives would go to answer the call of nature?
4) Why was it an embarassing thing if Sauda answered the call of nature?
5) Did others also spy on Muhammad's wives before Umar "recognized" Sauda?