Reference: al-Mulakhkhass al-Fiq.hee, v1/p264
Shaykh Saalih al-Fowzaan said:
And it is recommended [for the khateeb] to supplicate for the Muslims with that which comprises of goodness for their religion and their worldly affairs, as well as [to supplicate] for the Leader of the Muslims and their Rulers with success and uprightness.
[Know that] the supplication for the Rulers in the khutbah was something well-known amongst the Muslims, as he (the ruler) is responsible for their affairs. That is because the supplication for the Rulers of the Muslims with success and uprightness is from the way of Ahlus-Sunnah wal-Jamaa’ah, and abandoning it is from the way of the innovators.
al-Imaam Ahmed said: “If we had one supplication that was [guaranteed] to be answered [by Allaah], then we would have supplicated for the Ruler.”
[That is] because his [attaining] righteousness will lead to the righteousness of the Muslims.
And this Sunnah has been abandoned [so much so] that the people become surprised with the supplication for the Rulers, and they have evil suspicion of those who do so.