Is it better, when invoking blessings upon the Prophet (peace and blessings be upon him), to use the formula stated after the Tashahhud, or merely to frequently say: "O Allah, send blessings upon Muhammad"?

The best formula for sending blessings upon the Prophet, peace and blessings be upon him, is the Ibrahimiyah formula that he taught his companions. It is: "O Allah, send blessings upon Muhammad and upon the family of Muhammad, as You sent blessings upon the family of Ibrahim. Verily, You are Praiseworthy and Glorious. O Allah, bless Muhammad and the family of Muhammad, as You blessed the family of Ibrahim. Verily, You are Praiseworthy and Glorious." Another version includes the addition: "and his wives and his offspring." Al-Suyuti mentioned, quoting Al-Taj Al-Subki, that this method is the best way to send blessings upon the Prophet, peace and blessings be upon him, and that whoever recites it has certainly sent blessings. Al-Suyuti also stated that he used to employ another formula but then reverted to the exact wording of preference transmitted from the Prophet, peace and blessings be upon him.

Although the Ibrahimiyah formula is the best, reciting abundant blessings upon him using shorter formulas is better than reciting a smaller number using the Ibrahimiyah formula. This is due to the Prophet's, peace and blessings be upon him, sayings: "Whoever sends one blessing upon me, Allah will send ten blessings upon him," and "The people closest to me on the Day of Resurrection will be those who sent the most blessings upon me." Thus, increasing the number of blessings, even with a shorter formula, is superior.