It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Allah, the Blessed and Exalted, says: 'When I test My believing servant and he does not complain about Me to those who visit him, I release him from My captivity. Then I replace his flesh with better flesh and his blood with better blood, and then he begins (righteous) deeds afresh.'"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6548