Anas said God’s messenger liked what was at the bottom of the pot.* *Mirqat, iv, 381 has quite a long discussion of the meaning of thufl. It means 'dregs', or 'what sinks to the bottom'. The preference for the meaning in the translation above is said to "be either because what is in the bottom of the pot is cooked longer and so is better than the rest, or because the Prophet (ﷺ) liked others to be served first. It is also suggested that here thufl means tharid. Tirmidhi and Baihaqi, in Shu'ab al-iman, transmitted it.