Ibn ʿAbbās and other early authorities said: “Allah never sent a prophet without first taking from him the covenant that, if Muhammad were to be raised while that prophet was still alive, he would most certainly believe in him and support him; and He further ordered that prophet to exact the same covenant from his own community, that if Muhammad were sent during their lifetime they too would surely believe in him and aid him.”

The verse itself proves what they said, for His words — Exalted is He —
“When Allah took the covenant of the prophets” ⦗Āl ʿImrān 3:81⦘
extend to all of the prophets:
“Whatever I grant you of Scripture and wisdom, then there comes to you a messenger confirming what you have, you must indeed believe in him and support him.” ⦗Āl ʿImrān 3:81⦘

The first lām in this sentence is called the lām that introduces an oath, while the second is the lām that serves as the answer to the oath. When a statement combines both a conditional clause and an oath, and the oath is placed first, the response to the oath stands in the place of the response to the condition. The same structure appears in His, Exalted is He, words:
“If they are driven out, they will not go out with them; and if they are fought, they will not help them; and if they do help them, they will surely turn their backs, and thereafter they will not be helped.” ⦗Al-Hashr 59:12⦘

and likewise in His saying:
“Among them are those who pledged to Allah, ‘If He grants us of His bounty, we will surely give in charity and will surely be among the righteous.’” ⦗Al-Tawbah 9:75⦘