Musnad Ahmad 2:114

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from ‘Asim ibn ‘Ubayd Allah, from Salim, from ‘Abd Allah ibn ‘Umar, from ‘Umar, from the Prophet (peace and blessings of Allah be upon him): That he (Umar) asked his permission for the ‘Umrah, and he gave him permission. And he said: "O my brother, do not forget us in your supplication." And he said afterwards in Medina: "O my brother, include us in your supplication." ‘Umar said: "I would not love to have in exchange for it that upon which the sun rises, because of his saying: 'O my brother'."