In The Ring of Solomon, Bartimaeus puts on the titular artifact and summons the Spirit of the Ring, Uraziel. Fearing how the Spirit might react to seeing someone other than Solomon wearing the Ring, Bartimaeus takes Solomon's form.
It hadn’t escaped my notice that when Khaba had summoned him, the Spirit of the Ring had sounded slightly irritable not to see Solomon. Hence my clever disguise.
—The Ring of Solomon
Of course, the spirit of the Ring, being more powerful even than a marid, sees through his disguise with ease.
However, previously we have seen that even a mid-level djinni such as Bartimaeus can see on all seven planes, and on the seventh Bartimaeus's true form is clearly visible. Why would Bartimaeus have believed that his disguise could possibly fool such a powerful spirit?