Zevachim 40 - The Seven Sprinklings
The sacrifices that were brought inside the Temple Hall required seven blood sprinklings on the curtain and four applications to the Inner Altar. How do we know that these seven sprinklings were essential, and leaving out one of them would make the service invalid? - Because seven applications in all other places had this quality. And which are they? - The seven sprinklings of the preparation of the red heifer, and the seven sprinklings required for the purification of a spiritual leper.
How do we know that the four applications to the "horns" of the Inner Altar are also essential? - From the verse "so shall he do." This is an additional repetition of the requirement to do the prescribed service. Since we derived on the basis of the first repetition that the seven sprinklings toward the curtain are essential, we may derive from this repetition that the four applications to the horn are also essential.
Art: John Frederick Herring Snr - Galloping Horse in a Landscape