Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both), concerning the saying of Allah Almighty: "And they assigned to Allah a share of the crops and livestock He created, and said, according to their claim: 'This is for Allah, and this is for our partners.'" [Surah al-An'am: 136]
"They assigned to Allah" [Surah al-Ra'd: 16]: they assigned a share from their wealth and their fruits, and in the same way they assigned a share in the name of the devils and the idols. Then, if the share fixed for Allah fell short, they left it as it was; and if the share fixed for the devil fell short, they made up that shortfall from Allah's share. They did the same regarding watering, and likewise with what they assigned of livestock. Allah Almighty says: "Allah has not appointed any bahirah, sa'ibah, wasilah or ham." [Surah al-Ma'idah: 103]
Imam al-Shafi'i (may Allah have mercy on him) said that Allah Almighty revealed this verse concerning this: "Say: Bring your witnesses who testify that Allah has made this unlawful. And if they testify, do not testify with them." [Surah al-An'am: 150]
Here is a refutation of what they had made unlawful for themselves; Allah did not make it unlawful for them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19711