It is prescribed to send blessings upon the angels by saying “ ‘Alayhim al-salaatu wa’l- salaam (upon them be blessings and peace)” and “ ‘alayhim al-salaam (upon them be peace)”, because they are honourable, devoted worshippers and one of the creations of Allaah whom Allaah, may He be glorified and exalted, favoured over others as He says: 

“They [whom they call children of Allaah i.e. the angels, ‘Eesa (Jesus) __ son of Maryam (Mary), ‘Uzayr (Ezra)], are but honoured slaves”

[al-Anbiya’ 21:26] 

“But verily, over you (are appointed angels in charge of mankind) to watch you,

11. Kiraaman (Honourable) Kaatibeen —writing down (your deeds)”

[al-Infitaar 82:10,11] 

“In the hands of scribes (angels).

16. Honourable and obedient”

[‘Abasa 80:15, 16]

They have a particular status, virtue and honour, so it is prescribed to send blessings upon them, and there is no reason not to do so, rather it is prescribed. End quote.