Had the gain been within reach and the journey shorter, they would have followed you, but the distance seemed too long for them. And they will swear by Allah, “Had we been able, we would have certainly joined you.” They are ruining themselves. And Allah knows that they are surely lying.

Had it been a near gain (booty in front of them) and an easy journey, they would have followed you, but the distance (Tabuk expedition) was long for them; and they would swear by Allâh, "If we only could, we would certainly have come forth with you." They destroy their ownselves, and Allâh knows that they are liars.