﴿ وَإِذْ أَخَذَ ٱللَّهُ مِيثَٰقَ ٱلنَّبِيِّۦنَ لَمَآ ءَاتَيْتُكُم مِّن كِتَٰبٍۢ وَحِكْمَةٍۢ ثُمَّ جَآءَكُمْ رَسُولٌۭ مُّصَدِّقٌۭ لِّمَا مَعَكُمْ لَتُؤْمِنُنَّ بِهِۦ وَلَتَنصُرُنَّهُۥ ۚ قَالَ ءَأَقْرَرْتُمْ وَأَخَذْتُمْ عَلَىٰ ذَٰلِكُمْ إِصْرِى ۖ قَالُوٓا۟ أَقْرَرْنَا ۚ قَالَ فَٱشْهَدُوا۟ وَأَنَا۠ مَعَكُم مِّنَ ٱلشَّٰهِدِينَ ﴾

“AND, LO, God accepted, through the prophets, this solemn pledge [from the followers of earlier revelation]: "If, after all the revelation and the wisdom which I have vouchsafed unto you, there comes to you an apostle confirming the truth already in your possession, you must believe in him and succour him. Do you" - said He - "acknowledge and accept My bond on this condition?" They answered: "We do acknowledge it." Said He: "Then bear witness [thereto], and I shall be your witness.”

Please check ayah 3:82 for complete tafsir.