All perfect praise be to Allah, The Lord of The Worlds.
Sending blessings upon the Prophet (PBUH) is amongst the best Athkar (Remembrance of Allah) for every Muslim. In the Glorious Quran, Almighty Allah has commanded us to offer them where He said, "God and His angels send blessings on the Prophet: O ye that believe! Send ye blessings on him, and salute him with all respect."{Al-Ahzaab, 56}.
Sharia texts unite in clarifying the virtue of sending blessings upon the Prophet (PBUH) and its manner. Actually, there is no specific formula for that and all formulas, in this regard, comply with Sharia. The Prophet (PBUH) said: "Whoever sends salah upon me once, Allah (SWT) will send salah upon him tenfold."{Related by Muslim}.
A Muslim knows with certainty that both good and evil come from Allah, but if they are attributed to other than Him, then it is done as metaphor or causality. In addition, whoever holds the Prophet (PBUH) in honor does so because he (PBUH) is Allah`s slave and Messenger, and believing in him (PBUH) is a cause for every good. In addition, Istighfar (Seeking forgiveness) is a cause for forgiving sins; just as drinking water is a cause for removing thirst. However, all of this happens with the will and power of Allah the Almighty. In the Quran, some matters-upon which only Allah is capable-were attributed to Prophet Esa/Jesus(PBUH):"And (appoint him) an apostle to the Children of Israel, (with this message): "I have come to you, with a Sign from your Lord, in that I make for you out of clay, as it were, the figure of a bird, and breathe into it, and it becomes a bird by God’s leave: And I heal those born blind, and the lepers, and I quicken the dead, by God’s leave; and I declare to you what ye eat, and what ye store in your houses. Surely therein is a Sign for you if ye did believe;" {Al-Emran, 49}.
The formula referred to in the above question complies with the teachings of Sharia, and thus it is permissible. Allah the Almighty has stated in the Glorious Quran that Prophet Mohammad (PBUH) is the cause for repelling calamity and sparing the sinful and the disobedient punishment. Allah the Exalted said, " Remember how they said: "O God if this is indeed the Truth from Thee, rain down on us a shower of stones form the sky, or send us a grievous penalty."{Al-Anfaal, 33}. The Quran has also referred to repelling sickness and drought, in addition to other calamities. Believing in Prophets and divine messages is a cause for every good, in addition to repelling every evil and all pains. Allah, the Almighty said, "If only they had stood fast by the Law, the Gospel, and all the revelation that was sent to them from their Lord, they would have enjoyed happiness from every side. There is from among them a party on the right course: but many of them follow a course that is evil."{Al-Ma`idah, 66}.
Accordingly, the aforementioned formula is neither an act of disbelief nor polytheism, and it is prohibited to accuse Muslims with disbelief without a clear evidence. Rather, one must avoid this because disbelief and polytheism are very clear matters that can`t be proven on mere suspicion. Disbelief is the disavowal of the testimonies of faith and polytheism is the association of partners with Allah concerning good and evil. The status of the Prophet (PBUH) is tantamount to the status of Allah`s Message, and he (PBUH), in the sight of Allah, enjoys a sublime status; as confirmed in the Quran and the Sunnah. Insha`Allah, believing in the Prophet (PBUH) repels evil and brings good, and sending blessings upon him in the aforementioned formula is permissible. And Allah the Almighty knows best.