Sayyiduna Abu Rafi' (may Allah be pleased with him) said: "The Prophet (peace and blessings be upon him) prayed on Monday morning, Sayyidah Khadijah (may Allah be pleased with her) prayed in the last part of Monday, and Sayyiduna Ali (may Allah be pleased with him) prayed on Tuesday. So Sayyiduna Ali (may Allah be pleased with him) kept praying in secret for seven years and some months before anyone else prayed."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 948