Musnad Ahmad 27:1980

Uthman ibn Umar narrated to us, Isa ibn Hafs ibn Asim ibn Umar narrated to us, from Nafi, from Ibn Umar, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever bears its hardship and its severity with patience, I will be an intercessor or a witness for him on the Day of Resurrection."