The opinions of the Muslims who emphasize personal reasoning ( ahl al-ra'y ) have differed on this. Malik and al-Awza`i have said that the verse is not a part of the Qur'an and have unanimously forbidden its recitation in the obligatory [prayers] whether it be at the beginning of sura al-Hamd or the sura after it, or whether it (the prayer) is recited loudly or silently. Yes, they have allowed it's recitation in the supererogatory prayers.
As for Abu Hanifa, al-Thawri and their followers, they recited it at the beginning of the mother of the Qur`an (sura al-fatiha ) but they made it compulsory to recite it silently even when the prayers are recited loudly. This shows their agreement with Malik and al-Awza`i; maybe it proves it since we do not know the reason for reciting it silently in the prayers recited loudly except if it (the basmala ) is not a part of the mother of the book.
But al-Shafi`i recited it (the basmala ) loudly in the loud prayers and in the silent prayers he recited it silently. He counted it as a verse in the opening [chapter] of the book. This is [also] the opinion of Ahmad b. Hanbal, Abu Thawr and Abu `Ubayd. Differing reports have been reported from al-Shafi`i as to whether it is a verse in every chapter except for sura al-Bara'a (chapter 9) or whether it is not a verse except for the mother of the book. Both reports have been transmitted from him. But the researchers amongst his companions have agreed that the basmala is a part of all the chapters. They have interpreted away the two sayings reported from their Imam, al-Shafi`i.
As for us - the Imami community - we have agreed, in following the Imams of guidance of the family of the Prophethood - that it is a complete verse in the sab' al-mathani (the seven most repeated verses) and of every chapter in the great Qur`an except for [the chapter of] al-Bara'a (chapter 9) and that one who omits it intentionally in the prayer, his prayer becomes invalid whether it be an obligatory or recommended prayer. [We have also agreed that] it is essential to recite it loudly when the prayer is offered loudly and that it is recommended to recite it loudly in the silent prayers and that it is part of a verse of sura al-Naml (chapter 27). The textual proofs of our Imams in all of this are overwhelming and it's meaning successively transmitted .The contents are clear to refute their opposition to it (the basmala ) like the saying of Imam Abu 'Abd Allah al-Sadiq, peace be upon him, "What is it with them? They depend on the greatest verse in the book of God, the Almighty, the most Glorious, and they claim it is an innovation if they declare `In the name of God the most merciful, the most beneficent' openly."