All scholars agree that sending blessings upon Prophet Muhammad (peace and blessings be upon him) is part of the Prayer, even though they differ on the formula of how to send the blessings and whether it is an essential pillar, wajib (obligatory), or sunnah (recommended by the Sunnah). Whilst they differ on these two issues, they all mention that sending blessings upon Prophet Ibrahim (peace and blessings be upon him) is valid. Thus, we cannot say that it is not a part of the Prayer. We say that it is part of the Prayer, and that scholars differed on the strength of its obligation. Anyway, the Muslim should do his best to follow the example of the Prophet (peace and blessings be upon him) in the way he prayed.
The prominent Saudi Islamic lecturer and author, Sheikh MuhammadSaleh Al-Munajjid, states: Scholars differed concerning the ruling on sending blessings upon the Prophet (peace and blessings of Allah be upon him) in the Tashahud in Prayer. There are several opinions, one of which is that it is a pillar or an essential part of the Prayer, without which the Prayer is not valid. Others said that it is obligatory, and the third view is that it is sunnah and not obligatory.
The late prominent Saudi scholar, Sheikh MuhammadAl-`Uthaymeen, (may Allah bless his soul) regarded the third view as most likely to be correct. He said in his commentary on Zadal-Mustaqni`: Scholars differed concerning this matter and there were several opinions:
1 – The first opinion is that it is a ‘pillar’ of Prayer. This is the well-known view of the Hanbalischool of fiqh; that prayer is not valid without it. The evidence for that is the fact that the Companions asked the Prophet (peace and blessings of Allah be upon him): “O Messenger of Allah, we have learned how to send peace upon you, how should we send blessings upon you?” He said: “Say: Allahummasalli `ala Muhammad, wa `alaali Muhammad (O Allah, send blessings upon Muhammad and upon the family of Muhammad).” A command implies that it is obligatory, and the basic principle concerning an obligation is that it is fard, and if it is not done, then the Prayer is invalid. This is how the Muslim jurists (may Allah have mercy on them) settled this issue. However, if you think about this hadith, it is not obvious that sending blessings upon the Prophet (peace and blessings of Allah be upon him) is a ‘pillar’ of the Prayer, because the Companions were simply asking how to do it – “how should we send blessings upon you”? Therefore, the Prophet (peace and blessings be upon him) taught them how to do it. Hence, we say that the command in the word ‘Say’ does not mean that it is obligatory; rather, the purpose here was to teach them. If there is any other evidence which enjoins sending blessings upon the Prophet (peace and blessings be upon him) in Prayer, then it must be followed; if there is nothing except for this report, this does not indicate that it is obligatory, let alone that it is a ‘pillar’ of the Prayer.
2 – The second view is that it is obligatory, but that it is not a ‘pillar’, and the prostration of forgetfulness is required if it is omitted by mistake. They said: That is because his words, “Say: Allahummasalli `ala Muhammad, wa ‘ala aali Muhammad (O Allah, send blessings upon Muhammad and upon the family of Muhammad)” imply both a command and teaching. We cannot say that it is a ‘pillar’ of the Prayer without which it is not valid when it possible to understand the phrase to mean that it is an obligatory part of Prayer and not a ‘pillar’.
3 – The third view is that sending blessings upon the Prophet (peace and blessings be upon him) is a sunnah, and is not obligatory or a pillar of the Prayer. This view was narrated from Imam Ahmad. If a person deliberately omits it, his Prayer is still valid, because the evidence quoted by those who regard it as obligatory and those who regard it as a pillar of the Prayer does not clearly indicate either of the things suggested, so the basic principle is that it is not essential.
this is the view which is most likely to be correct because there is no other evidence apart from this report which the jurists (may Allah have mercy on them) quoted. We cannot regard an act of worship as being invalid because of evidence which may be understood as a command or as teaching.” (Al-Sharh al-Mumti`: 3/310-312)