Musnad Ahmad 1153:212

Yahya narrated to us, from Abd al-Rahman ibn Ammar—and he was trustworthy, and he is called Ibn Ammar ibn Abi Zaynab, a Medinan—he said: I heard al-Qasim ibn Muhammad, from Aisha, from the Prophet, peace and blessings of Allah be upon him: "The prayer in congregation is favored over the prayer of the individual by twenty-five [degrees]."