That Butler, Resting is Chapter 131.5 of the Kuroshitsuji manga. The extra chapter is a side story to the Luxury Liner Arc.
Summary
After returning to Phantomhive Manor, Ciel tells Sebastian that, just as he promised after Sebastian destroyed all Bizarre Dolls, he will get the next day off to recover from his injuries.
The following day, Finnian, Baldroy, Mey-Rin, Tanaka, and Snake enter Sebastian's room to wish him a good morning and to inquire about his health. To their surprise and shock, Sebastian is lying in his bed in his butler uniform. He says that he has only now realised that he does not own a pajama—making the other servants wonder in what he normally sleeps in.
Then, Baldroy gives Sebastian his clothes, however, they are slightly too small for him. Still, Sebastian decides to wear them but when he goes back to bed again and the others are gone, he cannot find any rest as he overhears his colleagues' words as they fail to do their respective jobs. Right after Sebastian thought that it would have been better if he had worked, Elizabeth enters his room, followed by Ciel.
Sebastian tells her that it's improper of a lady to go into a servant's room and that her mother would not be amused but Elizabeth does not mind. She tells him that she has heard of his state and brought him a get-well gift to cheer him up: a frilly nightgown and Bitter Rabbit slippers. Both Elizabeth and Ciel are delighted to see Sebastian wearing them.
Unable to ignore the outcries of the other servants, Sebastian ultimately spent his day off cleaning up after them. When he returns to his bedroom in the evening, he is surprisingly awaited by Ciel who wants to know how his day was. Sebastian tells him that he could not get any rest in the end before seeing that Ciel has brought him dinner. Then, he asks his master if he cooked the porridge to which Ciel replies that Tanaka and Snake prepared it and that his task was only to bring it to Sebastian.
Ciel tells Sebastian to get back to bed, and, afterwards, Sebastian asks him if he was "satisfied observing [his] pitiful appearance today." Ciel says that it was indeed amusing to see him in the clothes Elizabeth brought him.
Later, Sebastian eats his porridge—only to find out that it is "insanely sweet."
