A mu'allaq report is one in which one or more narrators are omitted from the beginning of the chain, so al-Bukhari says simply "Malik said..." or "Ibn 'Abbas said..." without naming his own teachers up to that point. These were not part of the strict conditions he set for the connected, authentic material of his Sahih; they appear mainly in his chapter headings and as supporting notes, and the great majority of them are in fact narrated with complete chains elsewhere in the book itself. Ibn Hajar's well-known rule is that a mu'allaq introduced with a definite verb ("qala", "rawa") is sound in al-Bukhari's judgement as far as the one it is attributed to, whereas one introduced with a weakening form ("yudhkaru", "ruwiya") requires checking and may be weak — though the weakening form does not by itself prove the report unsound. Ibn Hajar counted well over a thousand suspended reports in the Sahih and wrote an entire work, Taghliq at-Ta'liq, to supply full chains for them.
Q&A · Hadith
What is a mu'allaq (suspended) narration, and why are there some in Sahih al-Bukhari?
Evidence — the daleel
Ibn Hajar al-'Asqalani, Hady as-Sari (introduction to Fath al-Bari)Ibn Hajar al-'Asqalani, Taghliq at-Ta'liqIbn as-Salah, Muqaddimah fi 'Ulum al-Hadith
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.