Why do Shias say “Allahumma Salli Alaa Muhammad Wa Aali Muhammad”?

It has been established that the Prophet (S) himself had taught the Muslims how to invoke blessings on him. When the following noble verse

إِنَّ اللَّهَ وَمَلَائِكَتَهُ يُصَلُّونَ عَلَى النَّبِيِّ يَا أَيُّهَا الَّذِينَ آمَنُوا صَلُّوا عَلَيْهِ وَسَلِّمُوا تَسْلِيمًا

“Indeed Allah and His angels bless the Prophet. O you who have faith! Invoke blessings on him and invoke Peace upon him in a worthy manner,”[1]

was revealed, the Muslims asked the Prophet (S): “How should we invoke blessings (on you)?” The Prophet (S) said:

“Do not invoke ‘incomplete’ blessings on me.”

They again asked: “How should we invoke blessings on you?”He (S) replied: “Say:

"Allahumma Salli Alaa Muhammad Wa Aali Muhammad."

“O Allah! Bless Muhammad and the family of Muhammad.”[2]

The exceptionally high station of the Prophet’s family {al} (‘a) led Imam ash-Shafi‘i to compose the following famous poem:

"O members of the Household {Ahl al-Bayt} of the Messenger of Allah! (Our) love for you is an obligation, which God has revealed in the Qur’an.

Your lofty station such that if one does not invoke blessings on you (while offering prayers) one’s prayer will be of no avail.[3]

Notes:

[1] Surah al-Ahzab 33:56.

[2] Ibn Hajar, As-Sawa‘iq al-Muhriqah, 2^nd edition (Cairo: Maktabat al-Qahirah), Book 11, Chapter 1, p. 146 and a similar one in Jalal ad-Din as-Suyuti, Ad-Durr al-Manthur, vol. 5, commentary of Surah al-Ahzab 33:56 has been narrated byhadith scholars {muhaddithun} and compilers of Sahihs andMusnads such as ‘Abd ar-Razzaq, Ibn Abi Shaybah, Ahmad ibn Hanbal, al-Bukhari, Muslim, Abu Dawud, Tirmidhi, Nisa’i, Ibn Majah, Ibn Mardaway, from Ka‘ab ibn ‘Ujrah from the Holy Prophet (S).

[3] As-Sawa‘iq al-Mahriqah, Book 11, Chapter 1, p. 148; Shabrawi, Al-Ithaf, p. 29; Hamzawi Maliki, Mashariq al-Anwar, p. 88; Zarqani, Al-Mawahib; Sabban, Al-As‘af, p. 119.