So, we all know that Horus became Warmaster after the battle for Ullanor.
I know that Horus was the emperors favourite primarch:
For a thirty years he was the only Primarch to have been discovered. Friendship between the Emperor and Horus grew rapidly and the Emperor eventually trusted him enough to give him command of the entire force of the Imperium.
Horus flayed him alive with but a look and in that instant the Emperor realised how far his favoured son had fallen.
From the Lexicanum Article on Horus Lupercal
So, the question is not why the emperor made this decision. I am also aware that it was probably a process to decide which Primarch to make Warmaster. But what is the earliest point the Emperor mentions Horus as being the (probable) Warmaster?