Abu Hurayrah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Glad tidings to the slave who dies in a state of worshipping Allah and fully fulfilling the rights of his master; glad tidings to him, glad tidings to him."
In one narration al-Ramadi added these words: Whenever Umar (may Allah be pleased with him) passed by a slave, he would say: "O so-and-so, glad tidings to you of a double reward."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النفقات / 15811