That the house of every one is to him as his Castle and Fortress as well for defence against injury and violence, as for his repose; and although the life of man is precious and favoured in law; so that although a man kill another in his defence, or kill one per infortuntun [by misfortune], without any intent, yet it is felony, and in such case he shall forfeit his goods and chattels, for the great regard which the law hath of a mans life; But if theeves come to a mans house to rob him, or murder, and the owner or his servants kill any of the theeves in defence of himself and his house, it is no felony, and he shall lose nothing, and therewith agreeth 3 Edw. 3. Coron. 303, & 305. & 26 Ass. pl. 23. So it is holden in 21 Hen. 7. 39. every one may assemble his friends or neighbours to defend his house against violence: But he cannot assemble them to goe with him to the Market or elsewhere to keep him from violence: And the reason of all the same is, because domus sua cuique est tutissimum refugium [everyone’s house is his safest refuge; every man’s home is his castle].

Edward Coke (1552-1634) English jurist, politician

Semayne’s Case, 5 Rep. 91 (1604)
    (Source)

Published in Coke's Reports, Part 5 (1605), describing the case as (1604) Michaelmas Term, 2 James 1 In the Court of King’s Bench. This principle was further established as common law by Coke in The Institutes of the Laws of England, Part 3, ch. 73 (1628):

For a man’s house is his castle, et domus sua cuique est tutissimum refugium [and each man’s home is his safest refuge]; for where shall a man be safe, if it be not in his house?

For more on the earlier history of the phrase, see An Englishman's Home Is His Castle - Meaning & Origin Of The Phrase.