The statement that weak bases usually contain nitrogen is a matter of chemical definition and everyday observation, as most weak organic bases (such as amines and nitrogen heterocycles like pyridine or imidazole) derive their basicity from a lone pair of electrons on a nitrogen atom. Therefore, no citation is needed.
The claim falls under common chemical knowledge regarding the structure of weak bases (amines, nitrogenous bases) and does not require empirical verification from the retrieved literature.