Musnad Ahmad 1007:5

'Abd al-A'la narrated to us, from Ma'mar, from Yahya ibn Abi Kathir, from 'Abdullah ibn Abi Qatadah, from Abu Qatadah, that the Prophet of Allah (peace and blessings of Allah be upon him) forbade that any of it be mixed with anything else, but rather that each one of them be steeped separately.