The Holy Spirit
Whose Holy Spirit could it be, unless Allah is referring to Himself, which is highly unlikely considering His rants against those who would claim He is one of three? Could it be that He overheard Bahira talking about the Christian celebration of the Pentecost and the descent of the Holy Spirit on the twelve disciples and misunderstood who was, or were the recipients?
The first reference to the Holy Spirit is found in a diatribe against the Jews for refusing to accept the Prophet Muhammad as His latest, greatest and last Messenger.
2:87 We have indeed given Moses the Book, and after him We sent one Messenger after another. We also gave Jesus, son of Mary, clear signs (miracles) and strengthened him with the Holy Spirit. Do you, then, whenever a Messenger brings you what you do not desire, become puffed up with pride accusing some [of them] of lying and killing others?
2:88 They said: “Our hearts are sealed.” Nay, Allah has cursed them on account of their unbelief; for they have very little faith.
2:89 And when a book (the Qur’an) came to them from Allah confirming that which they had (the Torah), and though they used hitherto to pray for assistance against those who disbelieved; yet when there came to them what they already knew (that Muhammad would be sent as the Messenger of Allah) they disbelieved in it. So may Allah’s curse be on the unbelievers!
The second reference is in a revelation in which Allah substantially elevates the Prophet Muhammad above all other Messengers.
2:253 [Of] those Messengers, We have made some excel the others; to some of them Allah spoke (Moses and Muhammad) and He exalted some (Muhammad) many degrees [above the others]. We gave Jesus, son of Mary, clear signs and strengthened him with the Holy Spirit. Had Allah so willed, those who succeeded them would not have fought one another after they had received the clear signs. But they disagreed [among themselves]; some of them believed and some did not. Had Allah pleased they would not have fought each other, but Allah does what He wills.
The third reference to the Holy Spirit is in a revelation that on Judgement Day He will take credit for all the miracles perform by Jesus.
In the same breath, Allah reveals that it was the Holy Spirit which made it possible for Jesus to speak and be understood as soon as he was born, which is eerily reminiscent of what the Holy Spirit actually did for the apostles, allowing them to be understood by people who spoke a different language.
5:110 When Allah will say: “O Jesus, son of Mary, remember My Grace upon you and upon your mother, how I strengthened you with the Holy Spirit, so that you could speak to people in the cradle and as an old man; how I taught you the Book, the Wisdom, the Torah and the Gospel; and how, by My Leave, you created out of clay the likeness of a bird, and breathed into it, and then, by My leave, it turned into a bird. And you could heal the blind and the leper by My Leave and you could raise the dead by My Leave. And [remember] how I restrained the Children of Israel from harming you, when you brought them the clear signs (the miracles); whereupon the unbelievers among them said: “That indeed is nothing but manifest sorcery.”