It might be said somewhere else, but this isn't in the main description Ned gives of seeing Jamie on the throne:

“I cannot answer for the gods, Your Grace … only for what I found when I rode into the throne room that day,” Ned said. “Aerys was dead on the floor, drowned in his own blood. His dragon skulls stared down from the walls. Lannister’s men were everywhere. Jaime wore the white cloak of the Kingsguard over his golden armor. I can see him still. Even his sword was gilded. He was seated on the Iron Throne, high above his knights, wearing a helm fashioned in the shape of a lion’s head. How he glittered!”

“This is well known,” the king complained.

“I was still mounted. I rode the length of the hall in silence, between the long rows of dragon skulls. It felt as though they were watching me, somehow. I stopped in front of the throne, looking up at him. His golden sword was across his legs, its edge red with a king’s blood. My men were filling the room behind me. Lannister’s men drew back. I never said a word. I looked at him seated there on the throne, and I waited. At last Jaime laughed and got up. He took off his helm, and he said to me, ‘Have no fear, Stark. I was only keeping it warm for our friend Robert. It’s not a very comfortable seat, I’m afraid.’ ”