However, the hadith is authentic by virtue of its corroborating report:

Muslim narrated it on the authority of ʿAbd Allāh ibn ʿAmr, and I have documented it in my annotations on al-Ṭaḥāwiyyah (504).*

3306—(The first thing the people of Paradise will eat: the extra lobe of the fish’s liver).

Al-Ṭayālisī narrated it (2051), and through his route Abū Nuʿaym narrated it in al-Ḥilyah (6/252). He said: Ḥammād ibn Salamah narrated to us, from Thābit, from Anas, from the Prophet, peace and blessings be upon him; and he then cited it.

This is an authentic chain of transmission according to Muslim’s criteria.

It has also been reported through the route of Ḥumayd from Anas, as part of the account of the conversion to Islam of ʿAbd Allāh ibn Salām—may Allah be pleased with him. It includes the Prophet’s words, peace and blessings be upon him:

“...As for the first food that the people of Paradise will eat, it is the extra lobe of the fish’s liver...”

Al-Bukhārī narrated it (3329, 3938, and 4480); Ibn Ḥibbān (7161); al-Nasāʾī in al-Sunan al-Kubrā (9074); Ibn Abī Shaybah in al-Muṣannaf (13/125); Abū Nuʿaym in Ṣifat al-Jannah (336); Aḥmad (3/108 and 189); al-Baghawī in Sharḥ al-Sunnah (3769); al-Bayhaqī in al-Dalāʾil (2/528–529); and Abū Yaʿlā in his Musnad (3856), through several routes from Ḥumayd: Anas narrated to us that news of the Prophet’s arrival in Madinah reached ʿAbd Allāh ibn Salām, so he came to him to ask him about certain matters ... and he then cited the hadith in full.

It appears to me that the first hadith of Anas is in fact this very hadith of ʿAbd Allāh ibn Salām, except that Ibn Salām was not mentioned in the earlier version, in light of the well-known practice of the Companions relying upon one another in transmission.