Legislation of the Adhan

I saw in my dream a man selling either a bell or a horn. I asked to buy it, thinking one of these would serve to call the people to prayer, and he asked me why. When I explained, he said, "Should I not tell you something better than that?" I said, "Of course." So he taught me: when you want to call the people to prayer, say Allahu Akbar, Allahu Akbar, all the way to the end; the adhan as we now know it.

A vivid dream that so struck Abdullah ibn Zayd RA that he rushed to tell the Prophet ﷺ as soon as he woke.