It took me a few minutes to track down the original source of this. It is a paper by Dr George Walkden published in 2013 called "The status of hwæt in Old English" You can access the pdf from the link below [0].

The abstract reads:

>It is commonly held that Old English hwæt, well known within Anglo-Saxon studies as the first word of the epic poem Beowulf, can be ‘used as an adv[erb]. or interj[ection]. Why, what! ah!’ (Bosworth & Toller 1898, s.v. hwæt, 1) as well as the neuter singular of the interrogative pronoun hwa ̄ ‘what’. In this article I challenge the view that hwæt can have the status of an interjection (i.e. be outside the clause that it precedes). I present evidence from Old English and Old Saxon constituent order which suggests that hwæt is unlikely to be extra-clausal. Data is drawn from the Old English Bede, Ælfric’s Lives of Saints and the Old Saxon Heliand. In all three texts the verb appears later in clauses preceded by hwæt than is normal in root clauses (Fisher’s exact test, p < 0.0001 in both cases). If hwæt affects the constituent order of the clause it precedes, then it cannot be truly clause- external. I argue that it is hwæt combined with the clause that follows it that delivers the interpretive effect of exclamation, not hwæt alone. The structure of hwæt-clauses is sketched following Rett’s (2008) analysis of exclamatives. I conclude that Old English hwæt (as well as its Old Saxon cognate) was not an interjection but an underspecified wh-pronoun introducing an exclamative clause.

[0] https://kops.uni-konstanz.de/server/api/core/bitstreams/413d...