The only reference I could find is in Part 2, Chapter 10. Stave is explaining why people go back when their summoner dies - or why in some cases they remain in the Land. He says, "So it transpired three times for the ur-Lord, the Unbeliever." And he didn't mention the Staff in the previous text. Could this have been misinterpreted as all summons with the Staff?
Ah. I found it. It's when they are in the Hall of Gifts:
After a few steps, she asked, still indirectly, “How many times was Covenant summoned to the Land? I mean, before he and I came here together?”
“Four of which the Bloodguard had knowledge,” answered Stave.
“Who summoned him?”
Her companion had apparently accepted her fragmented state. He replied without hesitation, “The first summoning was performed by the Cavewight Drool Rockworm at Corruption’s bidding. The second, by High Lord Elena. The third, by High Lord Mhoram. In each such call, the necessary power was drawn from the Staff of Law. But the fourth was accomplished by the Giant Saltheart Foamfollower and the Stonedownor Triock, enabled only by their own desperation, and by a rod of lomillialor, of High Wood, gifted to Triock by High Lord Mhoram.”
Yes - it was a minor mistake. But I'm surprised we didn't catch it sooner - it's not as hidden as the Caer Caveral one (which I looked right at and didn't see).
And then the ravens pecked out his eyes.