Injecting electrical current is a fundamental operational mode of current-clamp pipettes in neuronal electrophysiology, making this statement trivially true by definition.
The claim states that current-clamp pipettes inject electrical current in neuronal recordings. This is a definitional property of current-clamp electrophysiology: the technique controls or passes current through the recording pipette into the cell. Because this is true by definition and everyday observation in electrophysiology, it represents common knowledge and does not require a citation.