Source for story of a Father's Patience

Question

A few people have mentioned a story (which they said was in a Chazal without knowing its location) recently to me about a husband who returns home after many years to find a young man in his home alone with the man's wife. He decides not to do anything until the next day where his wife tells him it's their own son.

Where is this mashal (or anything like it) located? I vaguely recall seeing such a story when discussing Reuven and Gad returning to their families after 14 years of conquering and apportioning Eretz Yisroel (though it's questionable whether they had rotations to return home or just stayed in the Holy Land the entire time)

Answer 1

It is in Sefer Chasidim 655:

מעשה בבן שכבד את אביו ביותר. א"ל האב אתה מכבדני בחיי תכבדני במותי. אני מצוך שתלין כעסך לילה אחד ועצור רוחך שלא תדבר לאחר פטירת אביו הלך לו למדינת הים והניח אשתו מעוברת והוא לא ידע. ועכב בדרך ימים ושנים וכשחזר בעיר בא בלילה ועלה לחדר שאשתו היתה שם שוכבת ושמע קולו של בחור שהיה מנשק אותה שלף חרבו ורצה להרוג שניהם. וזכר מצות אביו והשיב' לתערה. שמע שאמרה לאותו בחור בנה שאצלה כבר יש שנים רבות שהלך אביך מאצלי. אילו היה יודע שנולד לו בן כבר הגיע להשיא לך אשה. כששמע זה הדבר אמר פתחי לי אחותי רעיתי. ברוך ה' שעצר כעסי וברוך אבי שציוני לעצור כעסי לילה אחת שלא הרגתי אותך ואת בני. ושמחו שמחה גדולה ועשו משתה לכל העם הנמצאים ושמחו הרבה מאד.

Artscroll's Igeres HaRamban (p.34) loosely translates

Once there was a son who was extraordinarily respectful to his father. On his deathbed, the father said "My son, you honored me in my lifetime and now you must honor me after my death. I command you - if you should ever be overcome by anger, hold your anger in overnight." After his father's death, the son was forced to embark on a prolonged journey which took him to distant lands for tens of years. Unbeknownst to him, the wife he left behind was expecting his child. After his years of absence, the husband returned home unannounced, hoping to joyously surprise his wife. But as he approached his bed-chamber he saw his wife embracing a handsome young man, a stranger. The husband became fiercely jealous and reached for his dagger - when he suddenly remembered the pledge he made to his father: He must hold in his rage overnight. The next day he was shocked to discover that the young man in his wife's embrace was none other than his own son, the child that his wife had borne during his long absence. The man was thus saved from tragically slaughtering his own family.

Answer 2

I’ve read this story in the Artscroll book to the Iggeret HaRamban, and it’s a story that teaches that one should hold in his anger and not lash out.