Rashi on Genesis 37:29/רש"י על בראשית ל״ז:כ״ט

1וישב ראובן AND REUBEN RETURNED — When he (Joseph) was sold he had not been present, for it was his day (his turn) to go to attend to his father (Genesis Rabbah 84:19). Another explanation is: he had not sat with them at the meal because he was occupied with his sack-cloth and fast in penitence for having disturbed his father’s couch (Genesis Rabbah 84:19).