Allah said:

(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,

(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:

(...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.