Musnad Ahmad 489:2

Hushaym narrated to us, Ibn ‘Awn informed us, from al-Hasan, from ‘Iyad ibn Himar al-Mujashi‘i—and there was an acquaintance between him and the Prophet, may Allah bless him and grant him peace, before he was sent [as a Prophet]. When the Prophet, may Allah bless him and grant him peace, was sent, he [‘Iyad] presented him with a gift. He said: I think it was camels. He [the Prophet] refused to accept it and said: "We do not accept the *zabad* (froth/refuse) of the polytheists." He said: I said: "And what is the *zabad* of the polytheists?" He said: "Their aid, their gift."