Praise be to Allah, and peace and blessings be upon the Messenger of Allah.
Sending blessings upon the angels is permissible in general. However, it is not prescribed to add blessings upon them to the wording of the Tashahhud, because the Tashahhud consists of prescribed supplications and remembrances that should be adhered to as they were authentically reported from the Prophet ﷺ, without adding a wording that he did not report.
The Prophet ﷺ taught his Companions the wording of the Tashahhud and the blessings upon him. Therefore, the worshipper should restrict himself to the established wording and should not add blessings upon the angels to it on his own.
The fact that sending blessings upon the angels is permissible in other contexts does not mean that it should be added to the Tashahhud.
The worshipper says in the Tashahhud:
“Peace be upon us and upon the righteous servants of Allah.”
This greeting includes every righteous servant in the heavens and on earth, including the angels, peace be upon them.
Shaykh Salih al-Fawzan was asked about sending blessings upon the angels in the Tashahhud. He explained that the wording of the blessings prescribed in the Tashahhud should be restricted to what has been authentically reported, and that the angels are included in the worshipper’s statement:
“Peace be upon us and upon the righteous servants of Allah.”
It is not prescribed to specifically send blessings upon the angels within the Tashahhud in the same manner that we send blessings upon the Prophet ﷺ. Rather, the worshipper should restrict himself to the wording authentically reported from the Prophet ﷺ. The angels are already included in the greeting mentioned in our statement: “Peace be upon us and upon the righteous servants of Allah.”
And Allah knows best.