"Mary Had a Little Lamb" was a poem on Earth, written before the 20th century. The poem went:
"Mary had a little lamb, its fleece was white as snow, and everywhere that Mary went the lamb was sure to go."
In 1877, Thomas Edison recited "Mary Had a Little Lamb" to test his phonograph, making it the first audio recording ever. The second voice somehow became trapped on Edison's recording and was able to take over people's bodies by reciting the poem, transferring between bodies and tape. (AUDIO: Ghost in the Machine)
In his ritual to summon the Dæmon Azal, the Master chanted Mary Had a Little Lamb backwards: "Og ot erus saw bmal eht tnew yram taht, erehwyreve dna, wons sa etihw saw eceelf sti, bmal elttila dah yram!" (TV: The Dæmons)
Jo Grant recited "Mary Had a Little Lamb" to counter the Master's hypnotism. (TV: Frontier in Space)
Amber, under hypnosis from Mestizer, chanted guttural moans separated by long pauses in a séance to summon the Doctor's Cabinet of Light, which Honoré Lechasseur realised was her intoning Mary Had a Little Lamb, syllable by syllable. (PROSE: The Cabinet of Light)
Behind the scenes
The Master's speech during the ritual to summon Azal in The Dæmons is "Mary Had a Little Lamb" spoken backwards. In the novella The Cabinet of Light, Mestizer, whom Daniel O'Mahony intended to be the Master, employed the poem in her ritual.