We were with the Messenger of Allah ﷺ while he was distributing dates from the charity, with Hasan bin Ali on his lap. When he finished, the Prophet ﷺ lifted him onto his shoulder, and his saliva dripped onto the Prophet ﷺ. The Prophet ﷺ looked up and saw a date in his mouth, so he put his fingers in and took it out, then said, "Did you not know that charity is not lawful for the family of Muhammad?"
حدثنا عبد الرزاق أخبرنا معمر أخبرني محمد بن زياد أنه
كنا عند رسول الله ﷺ وهو يقسم تمرا من تمر الصدقة والحسن بن علي في حجره فلما فرغ حمله النبي ﷺ على عاتقه فسال لعابه على النبي ﷺ فرفع النبي ﷺ رأسه فإذا تمرة في فيه فأدخل النبي ﷺ يده فانتزعها منه ثم قال أما علمت أن الصدقة لا تحل لآل محمد
Its isnad is sahih, and it is in Jami al-Masanid 7:337, from this very place in the Musnad. Bukhari 3:280 and Muslim 1:295 narrated it similarly, in abbreviated form, through the route of Shubah, from Muhammad bin Ziyad, from Abu Hurairah. The Hafiz, in Fath al-Bari, pointed to this narration of Maamar found in Ahmad's Musnad, and did not attribute it to anyone else.