The following extract comes from a book by the Rev. Henry T. Cheever, Life in the Sandwich Islands, 1851 (p.107).
We arrived at half-past twelve the first night at a village where we thought to have stayed until day; but the kamaainas, inhabitants, were all away, and so we had to lay down as we were, supperless, (our man with food having fallen behind,) upon the round-stone floor of the meeting-house. Hard as it was, it would have been a grateful resting-place, but for the warfare of merciless fleas, ukulele, who, when they found what we were, and what a royal supper they might make on the blood of two haoles, set to so fiercely, that, after many vain struggles, we were forced to enter a nolo contendere, and leave the honours of the field to our insatiate foes.
We decamped about three, and rode on to Nuu, in Kaupo, where they hospitably entertained and lomilomied us, and I drowned several flying detachments of the ukulele tribe, by a bath in the sea.
This is the earliest European reference to the ukulele that I have found.