Suetonius, Caligula 51.2
adversus barbaros quoque minacissimus, cum trans Rhenum inter angustias densumque agmen iter essedo faceret, dicente quodam non mediocrem fore consternationem sicunde hostis appareat, equum ilico conscendit ac propere reversus ad pontes; ut eos calonibus et impedimentis stipatos repperit, impatiens morae per manus ac super capita hominum translatus est.
Although he also issued very many threats against the barbarians, when he was travelling in a chariot, and amid a close-packed column of men, through a narrow pass on the far side of the Rhine, someone said that there would be no little alarm if the enemy should appear from anywhere. Caligula immediately mounted a horse and rushed back to the bridges. When he found that they were crammed full of camp servants and baggage, he was impatient at the delay and was carried by hand over the men’s heads.