Which grammar books say it can be omitted? I wouldn't. It's something you might hear, but I would not write the sentence without it.
"It was you that told me to say so"
The issue of omission of that in this sentence varies depending on grammar books.
I wonder what you teachers think.
Personally I don't think it can be omitted.
Which grammar books say it can be omitted? I wouldn't. It's something you might hear, but I would not write the sentence without it.
"It was you who told me to say that" is also possible. Whether you use "who" or "that", I can't see a way of omitting it.
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.