Tafseer Ibn Kathir of AL HIJR [65]

65. ”Then travel for a portion of the night with your family, and you go behind them in the rear, and let no one amongst you look back, but go on to where you are ordered.'' 66. And We made this decree known to him that those (sinners) would be rooted out in the early morning. Allah tells us that His angels ordered Lut to set out after part of the night had passed. They told him to walk behind them, to protect them. Similarly, the Messenger of Allah would walk in the rear of the army on military campaigns, in order to help the weak and carry those who had no means of transport.

(and let no one amongst you look back,) meaning - when you hear the people screaming from their torment, do not turn around to look at them; leave them to face whatever punishment and vengeance is coming to them.

(but go on to where you are ordered.) - it is as if they had a guide with them to show them the way.

(And We made this decree known to him) meaning - We already told him about that.

(that those (sinners) would be rooted out in the early morning.) meaning in the morning, as in another Ayah:

(Indeed, morning is their appointed time. Is not the morning near) ï´¿11:81ï´¾