First Battle of Cato Neimoidia
Source: "Labyrinth of Evil" book.
From starwars.com blog, the following is the "save" description, neatly summarized:
During on Cato Nemoidia, Obi-wan lost/dropped his rebreather and Anakin found it. Eventually Anakin gave it back to Obi-wan when he needed it. (According to my recollection, reading LoE last year)
And here are the relevant quotes from the book:
On the fly he used the Force to call the thing to his left hand and realized that it was Obi-Wan's rebreather, which must have fallen from its utility pouch during the brief exchange with the unseen battle droids.
While all this was occurring, something else was afoot. Strands of some gauzy white substance were beginning to drift from the holed shipping container. Obi-Wan realized instantly what the substance was.
Taking one hand from the hilt of lightsaber, he began to fumble for the rebreather pouched on his belt, only to find it empty. "Stars' end," he cursed, more in disappointment than anger. Already beginning to feel woozy.
Anakin had his rebreather in his mouth as he approached the room in which Obi-Wan had held his own against better than fifty droids, all of which lay scattered about the room. A weaving, shuffling, staggering Obi-Wan was dealing with the last of them when Anakin entered. When the final droid collapsed, Obi-Wan aimed the blade of his lightsaber casually toward the floor and stood swaying in place, breathing hard but almost grinning.
"Anakin," he said happily. "How are you?" When Anakin went to him, Obi-Wan promptly collapsed in his arms. Anakin deactivated Obi-Wan's blade and inserted a rebreather into his mouth - - the same one that had ended up on the floor of the grotto.