Vitruvius 3.4.4 (tr. M.H. Morgan):
The steps in front must be arranged so that there shall always be an odd number of them; for thus the right foot, with which one mounts the first step, will also be the first to reach the level of the temple itself.
Gradus in fronte constituendi ita sunt, uti sint semper inpares; namque cum dextro pede primus gradus ascendatur, item in summo templo primus erit ponendus.
30 (tr. W. C. Firebaugh):
We had had enough of these novelties and started to enter the dining-room when a slave, detailed to this duty, cried out, "Right foot first." Naturally, we were afraid that some of us might break some rule of conduct and cross the threshold the wrong way; nevertheless, we started out, stepping off together with the right foot, when all of a sudden, a slave who had been stripped, threw himself at our feet, and commenced begging us to save him from punishment, as it was no serious offense for which he was in jeopardy; the steward's clothing had been stolen from him in the baths, and the whole value could scarcely amount to ten sesterces. So we drew back our right feet and intervened with the steward, who was counting gold pieces in the hall, begging him to remit the slave's punishment.
His repleti voluptatibus cum conaremur in triclinium intrare, exclamavit unus ex pueris, qui super hoc officium erat positus: "Dextro pede!" Sine dubio paulisper trepidavimus, ne contra praeceptum aliquis nostrum limen transiret. Ceterum ut pariter movimus dextros gressus, servus nobis despoliatus procubuit ad pedes ac rogare coepit, ut se poenae eriperemus: nec magnum esse peccatum suum, propter quod periclitaretur; subducta enim sibi vestimenta dispensatoris in balneo, quae vix fuissent decem sestertiorum. Retulimus ergo dextros pedes, dispensatoremque in atrio aureos numerantem deprecati sumus ut servo remitteret poenam.