Musannaf Ibn Abi Shayba 33:868

Husayn ibn Ali narrated to us, from Ja’far ibn Burqan, who said: Umar ibn Abd al-Aziz wrote: "To proceed: Verily, some people have sought the worldly life through the deeds of the Hereafter. And some of the storytellers (qussas) have introduced [the practice of] invoking blessings upon their caliphs and their commanders equal to their invoking of blessings upon the Prophet, may Allah bless him and grant him peace. So, when this letter of mine reaches you, command them that their blessings should be upon the Prophets, and their supplications should be for the Muslims in general, and let them abandon what is besides that."