Grading: This is a "jayyid" hadith (good and reliable).
Technical note / Defect ('illah): Because of Muhammad bin Sinan al-Qazzaz, this chain is "hasan" within corroborating chains and supporting reports, and this narrator also has corroboration (mutaba'ah).

Reference / Source: Abu Nu'aym narrated it in "Tarikh Asbahan" (volume 2, pages 356-357) by way of Abu Bishr Yahya bin Muhammad bin Qays, from Abu 'Amir al-'Aqadi, with this same chain.

Reference / Source: Musaddad reported it in his "Musnad" (according to "Ithaf al-Khiyarah": 3086), Imam al-Tabari in his "Tafsir" (volume 27, page 66) and al-Khara'iti in "I'tilal al-Qulub" (122) through the chain of 'Abd al-Rahman bin Nafi' al-Ta'ifi.
Details of the narration: Abu Hurayrah (may Allah be pleased with him) was asked about the verse "Those who avoid the major sins and immoral deeds, except al-lamam", and he said that "al-lamam" means looking (at a non-mahram), winking, kissing and bodily contact (without intercourse), but when the private part meets the private part, that is "zina", and ghusl becomes obligatory because of it.
Grading: Its chain is "hasan" within corroborating chains and supporting reports.

Technical note / Defect ('illah): Al-Hasan al-Basri narrated it from Abu Hurayrah (may Allah be pleased with him) ("Tafsir al-Tabari": 27/ 66-67 and "Shu'ab al-Iman": 6657-6658). In al-Tabari's narration the narrator thinks that he attributed it as "marfu'", while with al-Bayhaqi it is: from al-Hasan from the Prophet (peace and blessings be upon him), or from Abu Hurayrah from the Prophet (peace and blessings be upon him).
Details of the narration: "Al-lamam" means a single lapse into zina after which he repents and does not return to it, and likewise a single lapse into theft after which he repents and does not return to it.
Important point: Al-Hasan al-Basri's hearing from Abu Hurayrah is not established, so there is a break (inqita') in the chain.

Reference / Source: Al-Husayn al-Marwazi reported it in his additions (zawa'id) to Ibn al-Mubarak's "al-Zuhd" (1095), and Imam al-Tabari (27/ 67), as a saying of al-Hasan al-Basri himself, and this is more likely (al-ashbah).

Reference / Source: Imam al-Bukhari (6243) and Imam Muslim (2657) narrated from Ibn 'Abbas (may Allah be pleased with him) that he said: I have not seen anything more resembling "al-lamam" than what Abu Hurayrah narrated from the Prophet (peace and blessings be upon him): "Allah has written for the son of Adam his share of zina, which he will inevitably attain. The zina of the eye is looking, the zina of the tongue is speech, the soul wishes and desires, and the private part confirms all that or denies it."