Imam Al-Nawawi’s Riyad-us-Saliheen
Prohibition of Eating two Date-fruits Simultaneously
742. Jabalah bin Suhaim reported: We were with `Abdullah bin Az-Zubair (May Allah be pleased with them) in a time of famine, then we were provided with dates. (Once) when we were eating, `Abdullah bin `Umar (May Allah be pleased with them) passed by us and said: “Do not eat two dates together, for Messenger of Allah (sallallaahu alayhi wa sallam) prohibited it, unless one seeks permission from his brother (partner).”
[Al-Bukhari and Muslim].