Assalaamu alaykum. Is it permissible to use the expression: "Inna Allaaha Qaadirun 'alaa maa yashaa'u," meaning Allaah has power over whatever He wishes or wills. Please support your response with statements from the Quran, authentic Sunnah, or Salafi scholars. May Allaah reward you.

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

One should not utter the expression that you referred to in the question, "Inna Allaha Qaadirun 'ala maa yashaa" (Allah has power over whatever He wills), because Allah has power over whatever He wills and whatever He does not will, as He is over everything Omnipotent (Inna Allaha 'ala kulli shay'in Qadeer).

Shaykh Ibn ‘Uthaymeen clarified the Islamic prohibitions in this statement, as he said:

“There is an expression that some people say: “Inna Allaha Qaadirun 'ala maa yashaa;" they should not say this for the following reasons:

Firstly, because this is contrary to the absoluteness stated by texts (of the Quran and Sunnah), as the texts are absolute in this regard.

Secondly, because this statement may be understood to mean restricting the power of Allah to what He wills apart from what He does not will, whereas Allah has power over what He wills and what He does not will.

Thirdly, it may be construed to mean the belief of the Mu'tazilite Qadariyyah sect, who say, ‘Allah does not will the acts of the slave, as He has no power over them.

Therefore, we should say in an absolute manner what Allah said about Himself: “Inna Allaha 'ala kulli shay'in Qadeer” (Allah is over everything Omnipotent).

However, if the Power (Qudrah) of Allah is attributed to a particular action, then there is nothing wrong in restricting it with the Will of Allah, as in the saying of Allah (what means): {And He, for gathering them if He wills, is competent.} [Quran 42:29]; {if He wills} refers to {gathering} and not to the (competence), as Allah is competent over a thing whether He wills it or not, but {gathering them} only happens with His Will.

There is also the hadeeth about the story of a man whom Allah honored, and Allah said, “Walaakinni 'ala maa ashaa'u Qaadir" (But I have power over whatever I will) because He was speaking about a certain action; that is why He said, “Qaadir”, using the present participle, which indicates the occurrence of the action and not the adjective (As-Sifah Al-Mushabbahah) "Qadeer", which indicates being (eternally) characterized by omnipotence.”

Allah knows best.