It is narrated from Ubaidullah bin Abi Yazid that when a matter was asked of Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both), if it was mentioned in the Holy Qur'an, he would inform of it. If it was not in the Noble Qur'an but found in the hadith of the Messenger of Allah (peace and blessings be upon him), he would inform of it. And if it was not in the hadith either, he would look at the verdicts of Sayyiduna Abu Bakr and Umar (may Allah be pleased with them both). If he did not find the matter there either, then he would express his own opinion regarding it.