Musnad Ahmad 27:908

Musa ibn Dawud narrated to us, Ibn Lahi'ah narrated to us, from Khalid ibn Abi 'Imran, from Nafi', from Ibn 'Umar, that the Prophet, peace and blessings of Allah be upon him, used to say: "The Muslim is the brother of the Muslim; he does not wrong him, nor does he forsake him." And he used to say: "By the One in Whose hand is the soul of Muhammad, no two people have loved each other and then been separated, except by a sin committed by one of them." And he used to say: "The Muslim man has six rights of kindness over his brother: he should say 'Yarhamuk Allah' (may Allah have mercy on you) to him when he sneezes, visit him when he is ill, offer him sincere advice when he is absent, witness [his affairs], greet him when he meets him, answer him when he calls him, and follow him [in his funeral procession] when he dies." And he forbade a Muslim from shunning his brother for more than three [days].