Sayyiduna Abdullah bin Amr bin al-As (may Allah be pleased with them both) said that a man asked the Messenger of Allah (peace and blessings be upon him): O Messenger of Allah, the funeral of a disbeliever passes by us; should we stand up for it? He (peace and blessings be upon him) said: "Yes, stand up for it. You are not standing for it; rather, you are standing out of reverence for the One who takes the souls."
Sayyiduna Anas bin Malik (may Allah be pleased with him) said that he (peace and blessings be upon him) said: "I only stood up for the angel." And from Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him), from the Prophet (peace and blessings be upon him): "But we stand for the angels who are with it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6882