Where can I find the i.MX6 Reference Manual?

I am reading the Apalis datasheet for i.MX6, and it references the i.MX6 Reference Manual.
Where do I obtain this?

Thank you.

You can find it on NXP’s website, all documentation for i.MX6 Dual/Quad is listed on this page, and you can download reference manual directly from this link.

I assume that you are interested in Dual/Quad variant as these are used on the Apalis.