Miracles of saints in light of the Holy Quran and the Hadith
We have seen that the saints (Auliya) are those special servants of Allah Most High who spend every moment of their lives in the remembrance of Allah Most High. They have given up the desires of their flesh and have abandoned the pleasures of the flesh. They spend their entire energies in the propagation of Islam.
The Holy Quran mentions them time and again. This elite group of the Ummah will be high stations in the Day of Judgment and they will be granted many bounty. However, in this world itself, to express their high status, Allah Most High has granted them some special characteristics and qualities, which distinguish them from others.
Allah Most High says in the Holy Quran:
And surely We wrote in the Zabur (the Psalms) after (stating the) admonition that the inheritors of the earth (in the Hereafter) will be My pious servants. Surah Ambiya (21:105)
In the interpretation of this verse, some exegetes of the Holy Quran have written that the "land" meant here is the land of Paradise, while some others have said it means the land of this world. ‘Allama Ibn Kathir (May Allah shower His mercy on him) has written that the saints are the inheritors of the lands of both, the Paradise and this world.
Translation: Allah Most High is describing those things about which He has made the final decision for His pious servants that there are glad tidings for them in this world and the next and the land of this world and the hereafter is their inheritance.
When Allah Most High has made them the inheritors of the land, then out of his extreme benevolence, Allah Most High has granted them power and command, which is called as "Karamah"
It is one of the basic beliefs of the Ahle Sunnah that the miracles of the Saints are true, as given in the most basic tract on beliefs "Sharh Aqaaid Nasafi":
Translation: The miracles of the Saints are true. (Sharh Aqaaid Nasafi, Pg. No. 144)