Abu Hurairah (May Allah be pleased with him) said: The Messenger of Allah (PBUH) said, "Angels do not accompany the travellers who have with them a dog or a bell."[Muslim].
"Dog" here stands for that dog which is prohibited to keep. Dogs kept for the purpose of hunting and security do not fall in this category. Angels signify angels of mercy; otherwise we are attended all the time by the angels who record our deeds for us. Bell in this context refers to the bell which is placed in the neck of animals and rings when they move.