Allah said:

﴿فَإِذَآ أَمِنتُمْ فَمَن تَمَتَّعَ بِالْعُمْرَةِ إِلَى الْحَجِّ فَمَا اسْتَيْسَرَ مِنَ الْهَدْىِ﴾

(Then if you are in safety and whosoever performs the `Umrah (in the months of Hajj), before (performing) the Hajj, he must slaughter a Hady such as he can afford,)

That is, when you are able to complete the rites, so whoever among you connects his `Umrah with Hajj having the same Ihram for both, or, first assuming Ihram for `Umrah, and then assuming Ihram for Hajj when finished the `Umrah, this is the more specific type of Tamattu` which is well-known among the discussion of the scholars whereas in general there are two types of Tamattu`, as the authentic Hadiths prove, since among the narrators are those who said, "Allah's Messenger performed Tamattu`, and others who said, "Qarin" but there is no difference between them over the Hady.

So Allah said,

﴿فَإِذَآ أَمِنتُمْ فَمَن تَمَتَّعَ بِالْعُمْرَةِ إِلَى الْحَجِّ فَمَا اسْتَيْسَرَ مِنَ الْهَدْىِ﴾

(Then if you are in safety and whosoever performs the `Umrah (in the months of Hajj), before (performing) the Hajj (i.e., Hajj At-Tamattu` and Al-Qiran), he must slaughter a Hady such as he can afford,) means let him sacrifice whatever Hady is available to him, the least of which is a sheep. One is also allowed to sacrifice a cow because the Prophet slaughtered cows on behalf of his wives. Al-Awza`i reported that Abu Hurayrah narrated that Allah's Messenger slaughtered cows on behalf of his wives when they were performing Tamattu`. This was reported by Abu Bakr bin Marduwyah.

This last Hadith proves that Tamattu` is legislated. It is reported in the Two Sahihs that `Imran bin Husayn said, "We performed Hajj At-Tamattu` in the lifetime of Allah's Messenger and then the Qur'an was revealed (regarding Hajj At-Tamattu`). Nothing was revealed to forbid it, nor did he (the Prophet ) forbid it until he died. And somebody said what he wished (regarding Hajj At-Tamattu`) according to his own opinion.'' Al-Bukhari said that `Imran was talking about `Umar. It is reported in an authentic narration that `Umar used to discourage the people from performing Tamattu`. He used to say, "If we refer to Allah's Book, we should complete it,'' meaning:

﴿فَمَن تَمَتَّعَ بِالْعُمْرَةِ إِلَى الْحَجِّ فَمَا اسْتَيْسَرَ مِنَ الْهَدْىِ﴾

(...whosoever performs the `Umrah (in the months of Hajj), before (performing) the Hajj, he must slaughter a Hady such as he can afford,)

However `Umar did not say that Tamattu` is unlawful. He only prevented them so that the people would increase their trips to the House for Hajj (during the months of Hajj) and `Umrah (throughout the year), as he himself has stated.