Maysara reported,
I saw Ali 
drinking while standing. I said to him, "You drink while standing?"He replied, "If I drink standing, I have seen the Messenger of Allah ﷺ drink standing, and if I drink sitting, I have seen the Messenger of Allah ﷺ drink sitting."
Ahmad 916
Hasanحدثنا محمد بن فضيل عن عطاء بن السائب
عن ميسرة قال
رأيت عليا رضي الله عنه يشرب قائما قال فقلت له تشرب قائما فقال إن أشرب قائما فقد رأيت رسول الله ﷺ يشرب قائما وإن أشرب قاعدا فقد رأيت رسول الله ﷺ يشرب قاعدا
Its isnad is hasan, because Muhammad bin Fudayl's hearing from Ata bin Saaib came after the latter's mental confusion set in, as noted in al-Tahdheeb 7:205. Maysara is Ibn Yaqub al-Tuhawi, and the hadith has already appeared as no. 795 through the narration of Hammad bin Salama from Ata from Zazan, and it will also come again through his narration likewise at no. 1128, and it will come through the narration of Khalid bin Abdullah from Ata from Zazan and Maysara at no. 1125. These chains of transmission show that Ata heard it from both of them. The hadith of Maysara was not mentioned in Majma al-Zawaid, even though the hadith of Zazan was mentioned there. It will come through the narration of Khalid bin Abdullah from Ata from Zazan and Maysara at no. 1125, and through the narration of Hammad bin Salama from Ata from Zazan alone at no. 1128.