A source for which statement? I made a few. If you mean that they are thinking of relisting ARM, the linked article that this comment thread is under says that softbank is considering relisting ARM (already!).
If you want a source for my statement that big customers are designing their own CPUs then I could offer that Samsung is a current ARM customer that is known to be developing RISC-V cores:
https://www.electronicsweekly.com/blogs/mannerisms/dilemmas/...
https://www.androidauthority.com/samsung-risc-v-cpu-core-731...
I thought there were more direct sources back when the news came out about Samsung but don't see them now.
Western Digital has indicated many times that they will be transitioning 1 Billion cores per year to RISC-V:
https://www.youtube.com/watch?v=z48V8yMOFxA
It seems hard to find proof of what WD will be replacing with RISC-V but it seems extremely likely that its ARM, and here is one link that indicates ARM in a WD product:
https://www.hardwaresecrets.com/wd-my-cloud-3-tb-network-hdd...
We also know NVIDIA is replacing their Falcon GPU controller with a home-grown RISC-V core, but that will not displace ARM. They did indicate a desire for "rich OS" support, which they currently do with ARM. Here's their old presentation:
https://riscv.org/wp-content/uploads/2016/07/Tue1100_Nvidia_...
Looking around, it's easy to find companies supporting risc-v but hard to find direct statements indicating its use to replace ARM.
Anyway, given all the activity I think ARM sales representatives have a good view of their future prospects. They undoubtedly have some preliminary numbers on future ARM displacement by risc-v. If I were on an investors conference call with them (which they don't have because they're owned by softbank now?) I would ask directly about this very thing.