Surely those who swore allegiance to you O Prophet, indeed swore allegiance to Allah Himself. The Hand of Allah was above their hands. Now, the one who will break his pledge, will break it at his own peril, and the one who will keep his pledge that he has made with Allah, shall soon be given a great rewarded by Him.
Meaning:-
Allegiance: loyalty, faithfulness.
Pledge: promise, undertaking, vow, word of honor, oath.
Peril: danger, jeopardy, risk.