Tribute, Spoils, and Rulership (Kitab Al-Kharaj, Wal-Fai' Wal-Imarah)
Dawud :: Book 19 : Hadith 3051
Narrated Iyad ibn Himar:
I presented a she-camel to the Prophet (peace_be_upon_him). He asked: Have you embraced Islam? I replied: No. The Prophet (peace_be_upon_him) said: I have been prohibited to accept the present of polytheists.