Musannaf Ibn Abi Shaybah

Hazrat Ibn Umar (may Allah be pleased with them both) was asked about Mut'ah, so he replied that it is forbidden. He was told that Hazrat Ibn Abbas (may Allah be pleased with them both) gives a fatwa for its permissibility. Hazrat Ibn Umar (may Allah be pleased with them both) said, "Then why did he not proclaim this loudly during the time of Hazrat Umar (may Allah be pleased with him)?"