Isaiah 7:10–11 (King James Bible) moreover the LORD spake again unto Ahaz saying ask thee a sign of the LORD thy God
As Oliver Cowdery copied 2 Nephi 17:10–11 from 𝓞 into 𝓟, he initially skipped the word saying but later added it, perhaps while proofing 𝓟 against 𝓞 (the quill seems to be slightly sharper). The original manuscript (which is not extant here) probably had saying, the reading of the King James text.
Another example of Oliver Cowdery skipping the word saying is found in 1 Nephi 2:10. Here 𝓞 is extant and has saying, but Oliver omitted it as he copied from 𝓞 into 𝓟:
Nonetheless, there is a possibility that Oliver could have editorially added the saying here in 2 Nephi 17:10–11 since spake is usually followed by saying in the King James style. For an example of where Oliver added saying after spake (but inappropriately), see the discussion under 1 Nephi 7:1.
Summary: Maintain the word saying in 2 Nephi 17:10, the apparent reading of the original text (as well as the corresponding King James text).