Genesis 42:37
And Reuben spake unto his father, saying, Slay my two sons, if I bring him not to thee: deliver him into my hand, and I will bring him to thee again. And Reuben spake unto his father, saying, Slay my two sons, if I bring him not to thee: deliver him into my hand, and I will bring him to thee again.
The pledge comes after the brothers empty their sacks and find each man's money restored, a discovery that frightens them all, while Jacob is distressed at being bereaved of Joseph and Simeon and at the demand to give up Benjamin. The brothers are unwilling to agree to bring Benjamin to Egypt because the journey takes time while their families lack food, because no one wants to go alone or to leave the others in prison to face their father, and because they have seen their father's grief over Joseph's supposed death. They fear losing their lives or being enslaved, and fear that Benjamin could also be enslaved and their father would die feeling stripped of all his sons, so they choose to remain and suffer together rather than risk greater sorrow for their father through loss of his much-loved Benjamin.
In the verse Reuben asks for Benjamin to be entrusted to his care and promises to bring him back again, offering Jacob permission to kill Reuben's two sons if he fails and pledging their lives as a guarantee of Benjamin's safe return. The offer is a rash promise. It does not relieve Jacob's mind or ease his fear, and he refuses to let Benjamin go down with them to Egypt. Jacob refuses because Joseph is dead and Benjamin alone remains, and if harm comes to Benjamin on the journey it will bring Jacob's old age in sorrow to the grave.