From episode 1x02 "Birds of a Feather".
Source: IMDb
From episode 7x07 "The Return of Preston Giles".
From episode 5x02 "A Little Night Work".
From episode 5x11 "The Search for Peter Kerry".
From episode 6x03 "The Grand Old Lady".
From episode 8x15 "Tinker, Tailor, Liar, Thief".
From episode 11x08 "Crimson Harvest".