What is the reward gained from saying: "O Allah, bless our master Muhammad as many times as those who have blessed our master Muhammad, and as many times as the blessings that have been invoked upon the Prophet Muhammad"?

Sending blessings upon the Prophet, peace and blessings be upon him, is an act of worship with immense reward, because whoever sends blessings upon him once, Allah sends blessings upon him tenfold. For more details, please refer to the related fatwas.