Or HaChaim on Genesis 49:4/אור החיים על בראשית מ״ט:ד׳

1 פחז במים, "Hasty, like water, etc." Jacob accused Reuben of acting with haste, not trying to control his emotions and urges, failing to vanquish the spark of evil which was rooted within him. As a result of that failure he would be deprived of the three distinctions we have described i.e. birthright, priesthood and royalty.

2When Jacob added אל תותר, he hinted that although Reuben had displayed haste in doing something which held no advantage for him, he had not displayed the same haste when it came to doing something useful, i.e. repenting his mistake. Our sages in Baba Kama 92 say that it was Yehudah who triggered Reuben's eventual repentance. Although Bereshit Rabbah 84,19 states that G'd complimented Reuben on being the first human being who had repented [without having been accused by anyone as yet, Ed.], Reuben had not uttered a confession of his sin until he had heard about Yehudah doing this during Tamar's trial. Perhaps Reuben's tardiness in confessing his mistake was the reason that royalty was conferred on the descendants of Yehudah instead. It was Yehudah's reward for being the cause that triggered the final stage in Reuben's repentance. Reuben may have lost the priesthood to the descendants of Levi as a result of later developments as mentioned in Deut. 33,8 לאיש חסידך אשר נסיתו במסה, "Be with Your faithful one whom You tested at Massah" (when that tribe did not complain). Regardless of when Levi qualified for the priesthood, Reuben forfeited his claim to it already during Jacob's time. The words אל תותר mean that Reuben was deprived of all his erstwhile advantages over his brothers. Jacob transferred one of his former distinctions to Joseph, a second one to Yehudah, and he left the third one pending until G'd eventually accorded it to part of the tribe of Levi.

3Jacob said כי עלית to explain how it was possible that a son of his could commit such an act. He hinted that at the time (the wedding night) his thoughts had been confused as he had been under the impression that he had married and was sleeping with Rachel. In other words, he had actually slept in two beds, physically he had shared Leah's bed, whereas mentally he had considered himself in bed with Rachel. This is the reason Jacob spoke about Reuben having mounted משכבי אביך, the beds (pl) of your father. This was the original cause of Reuben eventually becoming guilty of his misdemeanour. When Jacob added the word אז, then, he indicated that the true defilement took place then; the first time Reuben desecrated his father's bed was at the time he was conceived in the wrong womb, Leah's instead of Rachel's. Jacob's משכבים were desecrated at that time, whereas Reuben personally had defiled Jacob's יצוע when he mounted the bed in Bilhah's tent.