Musannaf Ibn Abi Shayba 28:30

Waki’ narrated to us, he said: Sa’dan al-Juhani narrated to us, from Sa’d Abu Mujahid al-Ta’i, from Abu Mudillah, from Abu Hurayrah, he said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The supplication of the just leader is not rejected."