Allah's statement:

﴿فَإِنْ أُحْصِرْتُمْ فَمَا اسْتَيْسَرَ مِنَ الْهَدْىِ﴾

(But if you are prevented, sacrifice a Hady (animals for sacriface) such as you can afford,) was revealed in the sixth year of Hijrah, the year of the treaty of Al-Hudaybiyyah when the polytheists prevented Allah's Messenger from reaching the House. Allah revealed Surat Al-Fath (chapter 48 in the Qur'an) then, and allowed the Muslims to slaughter any Hady (animals for sacrifice) they had. They had seventy camels with them for that purpose. They were also permitted to shave their heads and end their Ihram. When the Prophet commanded them to shave their heads and end the state of Ihram, they did not obey him, as they were awaiting that order to be abrogated. When they saw that the Prophet went out after shaving his head, they imitated him. Some of them did not shave, but only shortened their hair. This is why the Prophet said:

«رَحِمَ اللهُ الْمُحَلِّقِين»

(May Allah award His mercy to those who shaved.)

They said, "What about those who shortened the hair'' He said in the third time, "And to those who shortened.'' Every seven among them shared one camel for their sacrifice. They were one thousand and four hundred Companions and were camping in the area of Al-Hudaybiyyah, outside the Sacred Area. It was also reported that they were within the boundaries of the Sacred Area. Allah knows best.

Being prevented from the House (Hasr) includes more than just being sick, fearing an enemy or getting lost on the way to Makkah. Imam Ahmad reported that Al-Hajjaj bin `Amr Al-Ansari said that he heard Allah's Messenger saying:

«مَنْ كُسِرَ أَوْ عَرِجَ فَقَدْ حَلَّ وَعَلَيْهِ حَجَّةٌ أُخْرَى»

(Whoever suffered a broken bone or a limb, will have ended his Ihram and has to perform Hajj again.) He said, "I mentioned that to Ibn `Abbas and Abu Hurayrah and they both said, `He (Al-Hajjaj) has said the truth'.'' This Hadith is also reported in the Four Collections. In the version of Abu Dawud and Ibn Majah, the Prophet said, "Whoever limped, had a broken bone or became ill...'' Ibn Abu Hatim also recorded it and said, "It was reported that Ibn Mas`ud, Ibn Az-Zubayr, `Alqamah, Sa`id bin Musayyib, `Urwah bin Az-Zubayr, Mujahid, An-Nakha`i, `Ata' and Muqatil bin Hayyan said that being prevented (Hasr) entails an enemy, an illness or a fracture.'' Ath-Thawri also said, "Being prevented entails everything that harms the person.''

It is reported in the Two Sahihs that `A'ishah said that Allah's Messenger went to Duba`ah bint Az-Zubayr bin `Abdul-Muttalib who said, "O Messenger of Allah! I intend to perform Hajj but I am ill.'' He said, "Perform Hajj and make the condition: `My place is where You prevent (or halt) me.'' Muslim recorded similarly from Ibn `Abbas. So saying such a condition for Hajj is allowed is based on this Hadith.

Allah's statement:

﴿فَمَا اسْتَيْسَرَ مِنَ الْهَدْىِ﴾

(...sacrifice a Hady such as you can afford) includes a sheep also, as Imam Malik reported that `Ali bin Abu Talib used to say. Ibn `Abbas said, "The Hady includes eight types of animals: camels, cows, goats and sheep.'' `Abdur-Razzaq reported that Ibn `Abbas said about what Allah said:

﴿فَمَا اسْتَيْسَرَ مِنَ الْهَدْىِ﴾

(...sacrifice a Hady such as you can afford)

"As much as one could afford.'' Al-`Awfi said that Ibn `Abbas said, "If one can afford it, then camels, otherwise cows, or sheep.'' Hisham bin `Urwah quoted his father:

﴿فَمَا اسْتَيْسَرَ مِنَ الْهَدْىِ﴾

(...sacrifice a Hady (animal, i.e., a sheep, a cow, or a camel) such as you can afford) `Depending on the price.'

The proof that sacrificing only a sheep is allowed in the case of being prevented from continuing the rites, is that Allah has required sacrificing whatever is available as a Hady, and the Hady is any type of cattle; be it camels, cows or sheep. This is the opinion of Ibn `Abbas the cousin of Allah's Messenger and the scholar of Tafsir. It is reported in the Two Sahihs that `A'ishah, the Mother of the believers, said, "The Prophet once offered some sheep as Hady.''

Allah's statement:

﴿وَلاَ تَحْلِقُواْ رُءُوسَكُمْ حَتَّى يَبْلُغَ الْهَدْىُ مَحِلَّهُ﴾

(...and do not shave your heads until the Hady reaches the place of sacrifice.) is a continuation of His statement:

﴿وَأَتِمُّواْ الْحَجَّ وَالْعُمْرَةَ لِلَّهِ﴾

(And complete, the Hajj and `Umrah for Allah.) and is not dependent upon:

﴿فَإِنْ أُحْصِرْتُمْ فَمَا اسْتَيْسَرَ مِنَ الْهَدْىِ﴾

(But if you are prevented, then sacrifice a Hady) as Ibn Jarir has erroneously claimed. When the Prophet and his Companions were prevented from entering the Sacred House during the Al-Hudaybiyyah year by the polytheists from Quraysh, they shaved their heads and sacrificed their Hady outside the Haram (Sacred) area. In normal circumstances, and when one can safely reach the House, he is not allowed to shave his head until:

﴿حَتَّى يَبْلُغَ الْهَدْىُ مَحِلَّهُ﴾

(.. .and do not shave your heads until the Hady reaches the place of sacrifice.) and then he ends the rituals of Hajj or `Umrah, or both if he had assumed Ihram for both. It is recorded in the Two Sahihs that Hafsah said, "O Allah's Messenger! What is wrong with the people, they have finished their Ihram for `Umrah but you have not'' The Prophet said,

«إنِّي لَبَّدْتُ رَأْسِي وَقَلَّدْتُ هَدْيِي، فَلَا أَحِلُّ حَتَّى أَنْحَر»

(I matted my hair and I have garlanded my Hady (animals for sacrifice), so I will not finish my Ihram till I offer the sacrifice.)