How to acknowledge a colleague's authorship of text in a syllabus

This sort of practice is quite common, and documents of this type are often treated as the "community property" of a department. This is a lot like authorship on a community wiki, where there is no expectation that the document is the original work of any author and there is the expectation that further authors may freely modify and contribute more text. It is therefore not plagiarism to use your colleagues notes as a base, any more than it is plagiarism to contribute to a Wikipedia article.

Often, these documents have no copyright or authorship markings, and are simply handled by custom and tradition. If you want to make the collective agreement more explicit and remove any doubt for those that come after you, then you should probably select a creative commons license (like CC0 or CC-BY) and put a small marking to that effect in the header or footer of the document. If you choose to formalize it in this way, however, you should explicitly get your colleague's consent as well, because you are moving the document beyond the informal method of relationship-based exchange in which it was given to you.

It is an extremely common and very accepted practice to copy other people's courses in their entirety or partially, with or even without asking for permission. At least in my field (computer science) people often put course material online with the intention of letting others use them freely. Unless explicitly written otherwise on a course website, I usually assume that it is ok to use any freely available material from there. Of course if someone actually gives you the material then there's no problem at all.

There is a big difference between journal papers, the sole purpose of which is to present new knowledge, and courses, where the goal is to teach known material to students. In the first case there is always a claim of originality, and if you are copying other work you are therefore commiting plagiarism. In the latter case, as long as there is no implicit claim of originality there is no problem.

People still like to be credited for their work, though. So for the sake of honesty and also giving credit, I usually tell my students where I got any significant part of the course material. If my course has a Web page, I state my sources there as well, and provide links to relevant notes or course Web pages when it's possible.