Some Muslims send peace and blessings upon the Prophet (peace be upon him) by saying:
O Allah! Send Your peace and blessings upon our Prophet Muhammad, the healing of hearts and the curing medicine. Is this permissible?
The best invocation of Allah's blessings upon the Prophet (peace be upon him) is AlSalatul Ibrahimiyyah (the second part of the Tashahhud) whose wording is: "Allahumma salli 'ala Muhammadin wa'ala ali Mihammadin kama sallayta 'ala Ibrahim wa'ala ali Ibrahim, innaka Hamidun Majid, Allahumma barik 'ala Mihammadin wa'ala ali Muhammadin kamma barakta 'ala Ibrahim wa'ala ali Ibrahim, innaka Hamidun Majid
(O Allah! Send Salah (Graces, Honours, Blessings, Mercy) upon Muhammad and upon the Family of Muhammad as You have sent Salah upon Ibrahim (Abraham) and upon the Family of Ibrahim, You are indeed Worthy of Praise, Full of Glory. O Allah send blessings upon Muhammad and upon the Family of Muhammad as You have sent blessings upon Ibrahim (Abraham) and upon the Family of Ibrahim, You are indeed Worthy of Praise, Full of Glory)."
This invocation of Allah's blessings upon the Prophet (peace be upon him) is the one authentically reported from the Prophet (peace be upon him), and it have different formulas. Any formula of them used in invoking blessings is permissible, as long as the formula is authentically reported from the Prophet (peace be upon him).