Riyad as-Salihin: The Book of the Etiquette of Sleeping, Lying and Sitting etc – Hadith No. 149 — Messenger of Allah (ﷺ) said, “It is not permissible for a man to separate… (Hadith No. 149)

'Amr bin Shu'aib on the authority of his father and grandfather reported: Messenger of Allah (ﷺ) said, "It is not permissible for a man to separate between two .
And in a version narrated by Abu Dawud "He should not sit between two men without their permission."

Explanation:

Reference: Riyad as-Salihin, The Book of the Etiquette of Sleeping, Lying and Sitting etc, Hadith No. 149