"And believe in what I have sent down," meaning: the Quran, "confirming what is with you, and do not be the first to disbelieve in it." He means: [in this, the tribes of] Qurayza and al-Nadir, because when the Prophet (peace and blessings of Allah be upon him) arrived among them in Medina, they disobeyed Allah, and they were the first among the Jews to disbelieve in it. "And do not exchange My signs for a small price," meaning: the signs with which Allah described Muhammad (peace and blessings of Allah be upon him) in their Book, which they concealed from the unlettered ones and the ignorant among the Jews. Those who were doing that were their scholars, such as Ka'b ibn al-Ashraf and his companions. They used to receive a tribute from the Jews every year, and that was the "small price"; they feared that if they followed the Prophet (peace and blessings of Allah be upon him), their tribute would vanish.