I can’t seem to find in the Book Of Mormon where Alma receive his Priesthood authority from? I don’t doubt he had it or our Heavenly Father wouldn’t have let the work continue at the waters of Mormon. I wondering from whom he would have received if from because from Chapter 17 to 18 it doesn’t seem he would have had the time to be able to have contact with Abinadi the prophet to receive it from him.
I believe that the question you ask has two parts. One of which we cannot answer clearly. The other, we can only partially answer.
The first question would be similar to asking “who ordained me an elder”? While I can tell you that about my personal history, we don’t really have much on Alma.
For he [King Noah] put down all the priests that had been consecrated by his father, and consecrated new ones in their stead, such as were lifted up in the pride of their hearts. Mosiah 11:5
Many take this to mean that Noah had the priesthood and then ordained new priests, thus handing down his priesthood to the wicked priests (of which Alma was one). I personally am not certain that is what this means. While it seems evident that Noah was of the Nephites and would have been ordained to some office of the priesthood, we don’t know if he held any keys to ordain new priests. He may have taken that authority all on his own. Therefore, Alma’s ordination in this case would have been null and void. But this is just speculation and questioning. For all we know, Noah did indeed have the proper keys.
However, since Alma was also a Nephite, he was probably also ordained by his own father earlier in life. This is evidenced by the fact that Noah and all the priests knew and understood Abinadi’s words. If they were not raised in the faith, then they would have had no idea what he was talking about.
So, the answer to this first part is probably “someone” ordained him originally.
The second part of the question is: “Who gave him both the keys and authority to baptize at the Waters of Mormon?” Well, we’ve already established that he was probably already ordained to the priesthood. That handles the authority. What about the keys?
And it came to pass that Alma, having authority from God, ordained priests; even one priest to every fifty of their number did he ordain to preach unto them, and to teach them concerning the things pertaining to the kingdom of God. Mosiah 18:18
It is important to understand that terms like “keys” vs “authority” may not have translated well from other languages. Because of that, I’m inclined to believe that the authority spoken of in this verse is indicative of keys as well as authority.
Also, the phrase “having authority from God” can simply mean that he had already been ordained, or it could mean that he had an angelic visitation. Some believe the latter. I tend to believe the former.
Now, it is important to remember that when I consider from whom I received my priesthood, I consider who ordained me to my latest office. It is not necessarily the person who ordained me as a deacon. So, who was the latest person that ordained Alma?
If you’re also interested in what happened after the Waters of Mormon episode, we read that King Mosiah made him head of the church in his kingdom.
Now king Mosiah had given Alma the authority over the church. Mosiah 26:8
This is certainly a departure from what we’re used to seeing in the Latter days. A prophet leaving his office? I do not believe he was no longer considered the earthly Head of the Church. But it seems he completely delegated the daily running of the church to Alma, almost entirely. The notion of delegation rather than recognizing his position that was already given him directly from God comes from the footnote on the word “authority”.
The most recent parallel we know of is when Elder Gordon B. Hinckley was a special assistant to the First Presidency. The First Presidency was getting older and their physical condition prevented them from performing their duties. Sometimes the Lord removes an impediment as he did with Isaiah. Sometimes, the Lord provides a help-meet as he did with Moses, giving him Aaron as a mouthpiece.
So, my best guess at understanding the line of authority in Alma’s life is, it seems that at that point the most recent authority given to him was what would be like ordaining Alma as an apostle today. And this was done under the hand of Mosiah.