There is no single definition, for what a word is, which is exactly my point. Some linguists even argue that "word" is inherently undefinable and refuse to use it as a category.

One common (but still ambiguous) definition is though, that a word is the smallest unit in a language that can stand on its own and conveys a meaning. By that definition, "Ruf .. an" is one word, as "an" is not a word by itself. It might not be too obvious, as "an" can also be a word by itself , just not in this context. Another example, where it's more obvious, is "innehalten". "Inne" is not a word, it has no meaning by itself, it cannot be used on its own, so in the sentence "halte kurz inne", "halte inne" is one word. Another example would be "Stelle etwas dar", where "dar" is obviously not a word by itself.

Fun fact: Verb literally means word in Latin, so saying they are the part of the same verb, but not the same word is kind of an oximoron.